US2718707A - Inclinometer - Google Patents

Description

S p 7, 1955 K. o. HEINTZ ETAL INCLINOMETER 2 Sheets-Sheet 1 Filed Dec. 31, 1953 INVENTORS. James H. Dunaway. BY Allen ,4- Chernosky,

Kar/ 0- He/nfz,

FIG. 5B.

p 27, 1955 K. o. HElNTZ ET AL INCLINOMETER 2 sheets-Sheet 2 Filed Dec. 31, 1955 United States Patent warmer rerun Karl O. Heintz, James H. Dunaway, and Allen A.

Chernoslry, Houston, Tex, assignors, by mesne assignments, to Essa Research and Engineering Company, Elizabeth, N. 5., a corporation of Delaware Application December 31, 1353, Serial No. 401,510

8 Claims. (Cl. 33215) The present invention relates to new and improved apparatus for determining the extent of inclination of an object from a vertical position. More particularly, the invention relates to improved apparatus for determining at the surface of the earth the inclination of a well logging instrument or similar device while it is in a well bore in the earth at any desired depth. I

In the drilling of oil wells, particularly when employing present day conventional rotary drilling methods, it is often difiicult if not impossible to cause the drill always to progress vertically downward at great depths into the earth. Also in conventional drilling practice, it sometimes becomes desirable to divert the drill purposefully away from a vertical direction. In either case it becomes desirable frequently to measure the direction and amount of inclination of the well bore from the vertical throughout its length so that the position of the bottom of the well, or of any intermediate portion of the well bore, relative to the mouth of the well may be calculated with reasonable accuracy. To this end, many forms of apparatus and combinations of apparatus elements have been devised to yield the desired information.

It is one object of the present invention to provide an improved apparatus for determining inclination from the vertical. It is another object of the present invention to provide an improved apparatus which may be used in combination with orientation devices to determine both the direction and amount by which a well bore or a device in the well bore deviates from the vertical. It is a further object of the invention to provide an apparatus which will record this information as the well bore is traversed by the apparatus.

Stated briefly, the apparatus of the present invention comprises a free pendulum, and a smaller, yieldably restrained pendulum which is carried upon the free pendulum. When the apparatus is supported with its central axis in a vertical position, the free pendulum is free to rotate around this central axis carrying the entire restrained pendulum with it. In the aforementioned vertical position, the pendulum arm of the restrained pendulum is also movable around a vertical axis but the restraining element associated with this smaller pendulum causes the small pendulum arm to take up a predetermined position arcuately spaced around the central axis from the center of gravity of the free pendulum. When the central axis of the apparatus is tilted from the vertical position, the center of gravity of the free pendulum moves into a position below the tilted central axis and the restrained pendulum arm moves arcuately, relative to the center of gravity of the free pendulum, from the aforementioned predetermined position toward the direction of tilt of the central axis.

A preferred embodiment of the apparatus of the present invention also comprises at least two mirrored surfaces associated, respectively, with the free pendulum and with the restrained pendulum, a light source adapted to revolve around the central axis of the apparatus,

2,718,707 Patented Sept. 27, 1955 ice a photosensitive cell adapted to revolve in fixed relation to the light source and so positioned as to receive reflected light from each of the mirrored surfaces during some portion of each revolution around the central axis, and means for revolving the light source and photosensitive cell around the central axis at a uniform rate.

The nature and objects of the invention can be more fully understood from the ensuing description when considered in conjunction with the accompanying drawings in which:

Figure 1 is an elevation view, partly in section, of one embodiment of apparatus employing the present invention;

Figure 2 is a sectional view along the line IIII of Figure 1;

Figure 3 is a sectional view along the line IIIIII of Figure 1;

Figure 4 is a sectional view along the line IVIV of Figure 1;

Figures 5A and 5B represent the type of records produced by recording elements employed with the apparatus of the present invention;

Figure 6 is an elevation view, partly in section of another embodiment of apparatus employing the present invention;

Figure 7 is a plan view of the apparatus of Figure 6;

Figure 8 is an elevation view, partly in section, of a preferred embodiment of apparatus employing the pres ent invention; and

Figure 9 is a sectional view along the line IXIX of Figure 8.

In the following description of the several figures of the drawing, it is assumed, for purposes of simplification of description and definition, that the apparatus, as described, is held or supported with its principal central axis in a vertical position. It will be appreciated, however, that in normal operation this principal central axis may be tilted away from vertical whereupon certain elements of the apparatus will move relative to this central axis.

Referring first to Figures 1 and 2, the apparatus of this invention may be suitably enclosed within a housing 10 which will generally be cylindrical although it may, of course, be of any other desired shape. Within the housing 10 are a pair of conventional ball or roller bearings 11 Whose outer race members may be secured against movement with respect to housing 10 by any appropriate means, such as the set screws 12. Fitted concentrically Within the inner race members of bearings 11 is an elongated, usually cylindrical, support member 13 whose central longitudinal axis 14 is preferably coaxial with respect to the central axis of housing 10. For purposes of the present description, it will be assumed that both of the aforementioned axes are coaxial and may be referred to indiscriminately by the reference numeral 14. The lower end of support member 13 may be provided with a plate, or equivalent element, 15 adapted to bear against a suitable thrust bearing, such as the ball thrust bearing 16, carried on plate 17 within housing 10.

Affixed to one side of support member 13 is a major mass 18. The size, shape, and placement of mass 18 upon support member 13 will depend to a large extent upon the mass symmetry of support member 13 and upon the distribution of other masses which may be mounted upon member 13. Since the primary purpose of mass 18, when combined with support member 13, is to form a free pendulum adapted to rotate freely around axis 14, it will be appreciated that the size, shape, and placement thereof should be chosen so that the effective center of gravity of these elements is adjacent the point CG in Figures 1 and 2. Hereinafter and in the appended claims, this effective center of gravity will be referredto generally as the center of gravity of the major mass 18. From the foregoing, it will be appreciated that although the mass 18 has been shown exterior of the support member 13, the mass may, if desired, be mounted on the interior of the support member so long as the effective center of gravity thereof is eccentric (i. e. spaced radially) with respect to central axis 14.

It will be apparent to workers in the art that, when the central axis 14 is vertical, the support member 13 carrying mass 18 is free to rotate around axis 14. However, when the apparatus is tilted so that, for example, the upper end of axis 14 is tilted to the left within the plane represented by the surface of the paper on which Figure 1 is drawn, mass 18 will rotate around axis 14 until its center of gravity is in the same plane as axis 14 and in the direction of tilt of said axis. In other words, with the as sumed direction of tilt mentioned above, mass 18 would move arcuately through 180 degrees from the position shown in the drawing. Movement of mass 18 would, of course, cause support member 13 to turn about its central axis.

Carried upon the upper end of support member 13 by a plate, or equivalent supporting structure, 19 is a restrained pendulum or Weighted arm assembly comprising a minor mass 20 affixed to the outer extremity of a lever arm 21. The weight of minor mass 26 is but a small fraction of the weight of mass 18 and need be only large enough to impart a turning moment to lever arm 21. As may be seen from Figure 1, the inner extremity of lever arm 21 is secured to a vertical shaft 22 so that arm 21 is held in a horizontal position when axis 14 and shaft 22 are vertical. In other words, the lever arm 21 is arranged in a plane which is substantially perpendicular to the axis of the shaft 22. Shaft 22 is mounted for substantially frictionless rotation on jeweled pivot bearings 23 and 24, or the equivalent. Pivot bearings 23 and 24 are, in turn, held in place relative to plate 19 by brackets 25 and 26 respectively. Resilient spring means, such as a flat spiral hair spring 27 encircling shaft 22, is anchored at its outer end to bracket 25 and thence through plate 19 to support member 13. The inner end of spiral spring 27 is operatively connected to lever arm 21 by direct attachment to shaft 22 on which lever arm 21 is secured.

It will be appreciated that spring 27 may be so adjusted that, when shaft 22 is vertical, arm 21 and mass 20 will always come to the resting or reference position represented by the solid-line outline of these elements in Figure 3. On the other hand, when the upper end of shaft 22 is tilted to the right in the plane of the paper on which Figure 1 is drawn, mass 20 will produce a turning moment on arm 21, against the yieldable restraining action of spring 27, and cause the mass 20 and arm 21 to seek the position represented by the broken-line outline of these elements shown in Figure 3. The movement of mass 20 from the resting, or reference, position around the axis of shaft 22 will be proportional to the amount of tilt of shaft 22.

It is important to the operability of the apparatus of the present invention that, when the central axis 14 is in a vertical position, the center of gravity of mass 20 should be spaced arcuately around axis 14 with respect to the center of gravity CG of the mass 18. Accordingly, plate 19 is preferably secured to support member 13 by a plurality of means, such as screws 28 threaded into support member 13 and passing through arcuate slots 29 provided in plate 19. By suitable initial placement of plate 19 upon support member 13 and slight rotary adjustment of plate 19 around axis 14 before tightening screws 28, the normal resting position of mass 20 may be spaced arcuately around axis 14 from center of gravity CG in the relative position represented in Figures 1, 2, and 3. In general, the normal resting position of the center of gravity of minor mass 20 should be spaced arcuately around axis 14 from the center of gravity CG of major mass 18 by an amount less than 180 degrees and preferably not less than about 90 degrees.

In the drawing of Figures 1 and 3 the longitudinal axis of shaft 22 has been represented as coaxial or coincident with the axis 14 of support member 13. It is to be noted, however, as will become apparent from subsequent description of the embodiment shown in Figures 8 and 9, that the axis of shaft 22 need not be coincident with axis 14 so long as the two axes are substantially parallel to each other. Stated broadly, the lever arm 21 should move in a plane which is substantially perpendicular to the axis 14, and the axis of shaft 22 should be substantially perpendicular to the plane in which lever arm 21 moves.

The apparatus of the present invention is readily adaptable to yield inclination indications at a remote distance from the apparatus thus far described. To this end there may be combined with the inclination responsive elements described above photoelectric means, similar to the means disclosed by A. B. Hildebrandt in United States Patent 2,614,334 issued October 21, 1952. Thus, the mass 20 may be provided with a plane mirrored surface 20a and there may be aflixed to the upper surface of plate 19 one or more plane mirrored surfaces 30. In the form of apparatus shown in Figures 1 and 3, the respective planes of mirrored surfaces 20a and 30 should be substantially horizontal when axis 14 is vertical.

Mounted within housing 10 upon a rotatable shaft 31 is a table member 32. A bracket 33 carrying an electric lamp within a lamp shield 34 is secured upon table member 32 in a position such that light from the lamp may pass through a slit 35 in shield 34 and shine upon the mirrored surfaces 20a and 30 when shaft 31 is rotated. Also secured to table member 32 is a bracket 36 carrying a photosensitive cell 37 in a fixed relation to the light source 35. Photosensitive cell 37 is positioned relative to light source 35 and mirrored surfaces 20a and 30 so that during some portion of each revolution of shaft 31 and table member 32 said cell will receive light reflected from each of the mirrored surfaces. As may be seen from Figure 1, the shaft 31 is preferably mounted coaxial with respect to the central axis 14. A plurality of slip rings 38 carried upon shaft 31 provide suitable sliding contacts for electrical connections leading to and from the photosensitive cell 37 and the source of light within light shield 34. Shaft 31 may be driven at a uniform rate by a motor 39 mounted within housing 10. As will be apparent to workers in the art, motor 39 may be a clock-work motor or a small electric motor and may include a gear reduction box (not shown) between motor 39 and shaft 31.

Referring now to Figures 5A and 5B there are shown, respectively, two graphs such as would be produced by the apparatus of the present invention when employed with the elements shown and described in conjunction with Figure 5 of the aforementioned Hildebrandt Patent 2,614,334. In the drawing of Figures 5A and 5B of the present disclosure, it is assumed that a single, relatively narrow, mirrored surface 20a is arranged upon minor mass 20 and that two closely spaced mirrored surfaces 30 have been mounted upon plate 19 in the relation shown in plan view in Figure 3. It is further assumed that the light source 35 and photosensitive cell 37 scan these mirror surfaces in a clockwise direction. When the axis 14 of the apparatus is held in a vertical position, passage of the light source and photosensitive cell over mirrored surfaces 30 produces the two kicks or responses 40 shown in Figure 5A. Passage of the light source and photosensitive cell over mirrored surface 20a in the course of revolution of table member 32 produces the single kick 41. Since the rate of movement of the recording chart and the rate of scanning of the mirrored surfaces are both held constant, the distance between kicks 40 and 41 are proportional to the arcuate spacing of mirrored surfaces 30 from mirrored surface 20a. Continued revolution of table member 32 causes the light source and photosensitive cell to pass once again over the mirrored surfaces 30' and produces the kicks 42.

When the axis 14 of the apparatus is tilted away from vertical, the mass 18 moves to a position such that its center of gravity is below the tilted axis, and this motion causes mirrored surfaces or indicants 30 likewise to move. Under these conditions of tilt, the mass 20 carrying mirrored surface or indicant 20a moves arcuately toward mirrored surfaces 30 by an amount proportional to the inclination of axis 14. As shown in Figure 53, this action causes the kick 41a, produced by reflection of light from mirrored surface 20a, to move closer to the kicks 40a, produced by reflection of light from mirrored surfaces 30; and the distance between kicks 40a and kick 41a is proportional to the amount of tilt applied to axis 14. As will be apparent to workers in the art, the actual relation between the angle of tilt and the amount of separation between the recorded kicks 40a and 41a may readily be determined from a calibration chart provided for each inclinometer and recorder assembly.

Turning now to Figures 6 and 7 there is shown a modifled embodiment of the apparatus of the present invention adapted for direct reading. In this embodiment, the housing 10 is provided with a flat base plate 43 whose outer plane surface is perpendicular to the central axis 14. The upper end of housing 10 may be closed by a transparent window 44 which may be held in place by appropriate internal and external rings 45 and 46, respectively.

Secured within housing 10, by means such as set screws 12, is a conventional tapered roller bearing assembly 11a which may serve not only as a swivel hearing but also as a thrust bearing. Secured centrally within the inner race element of the bearing assembly 11a is a light weight support member 13 having a major pendulum mass 18 secured therein with the center of gravity of said mass eccentric with respect to the axis 14. A restrained pendulnm or weighted arm assembly comprising the plate 19, minor pendulum mass 20, lever arm 21, pivot shaft 22, pivot bearings 23 and 24, pivot support brackets 25 and 26, and spiral hair spring 27 is mounted upon the upper end of support member 13. As in the embodiment illustrated and described in conjunction with Figure 1, the plate member 19 is preferably affixed to support member 33 by means of screws 28 which threadably engage support member 13. Arcuate slots, similar to the slots 29 shown in Figure 3, may be provided in plate 19 although these slots are not visible in Figures 6 and 7. When the apparatus of Figures 6 and 7 is assembled, plate 19 may be mounted upon support member 13 and its position adjusted so that, when central axis 14 is vertical, the center of gravity of mass is spaced arcuately around said axis a suitable amount relative to the center of gravity of mass 18.

In the embodiment of apparatus shown in Figures 6 and 7, the outer extremity of lever arm 21 may be provided with a pointer 47, or other suitable indicant, which may cooperate with a calibrated indicant scale 48 carried upon a dial plate 49 which may be suitably afiixed to plate 19 or support member 13. It will be apparent that lever arm 21 must be spaced slightly above dial plate 49 and also that the perimeter of dial plate 49 must not drag against the walls of housing 10. Indicant scale 48 may be calibrated to indicate directly the degrees of inclination of central axis 14.

Referring now to Figures 8 and 9 there is shown a preferred embodiment of apparatus in accordance with our invention, this embodiment of apparatus being particularly adaptable to use as a well bore inclinometer. Besides the elements shown in the drawing, the housing 10 may include suitable apparatus, such as a recording magnetic compass or a gyroscope for azimuthal orientation of the apparatus. Orientation apparatus of the above indicated type is well known in the prior art and does not form a direct part of the present invention. Accordingly, such orientation apparatus will not be described herein.

Suitably aflixed within housing 10, substantially perpendicularly with respect to the central longitudinal axis 14 thereof, is a mounting plate 50. Arranged coaxially with respect to axis 14 and anchored'centrally in plate 50 as by means of transverse pin 51, is a vertical shaft 52 depending below plate 50. Journaled upon shaft 52, as by upper and lower precision ball bearings 53 and 54, respectively, is a major pendulum mass 18a. Mass 18a is so shaped that its effective center of gravity CG is eccentric with respect to shaft 52 and central axis 14, and it is supported for rotation on shaft 52 by ball thrust bearing 55 and a nut, or other retaining element, 56 fixedly engaging shaft 52.

Depending from and suitably aifixed to the lower end of mass 18a is a cylindrical cup-like member 57 which functions, in part, as a support member analogous to the support member 13 shown in Figures 1 and 6. The lower end of cup or support member 57 is closed by an inwardly depressed bottom portion 58 which thereby forms in the lower end of cup member 57 an annular cavity 59. Rigidly afiixed to the lower end of shaft 52 as by means of nut 60 is an inverted cup 61 having the rim thereof concentric in the annular cavity 59. The cup-like support member 57 is partially filled with a viscous liquid, such as a high viscosity-index lubricating oil or other similar viscous liquid whose viscosity is not appreciably changed by changes in temperature. It will be appreciated that the inverted cup 61 and the cup-like support member 57 cooperate, through the medium of the viscous liquid contained therein, to provide a viscous drag capable of damping oscillatory movement of mass 18a around shaft 52 and relative to housing 10.

So that the fluid contained Within cup 57 will not be readily spilled therefrom, the upper end of cup 57 may be closed by a plate 62 having a central opening adapted to pass shaft 52 as shown in the drawing. A skirt 63 depending from plate 62 around the periphery of the aforesaid opening may also be provided to prevent spillage of the fluid out of cup 52.

Depending from the lower end of cup-like support member 57 is a restrained pendulum assembly, designated generally by the numeral 64. In a preferred embodiment of our invention, the restrained pendulum assembly 64 is provided by suitable, and minor, modification of a commercially available direct current microampere panel-type electric meter whose outer case is designated by the numeral 65. The electrical terminals 66a and 66b of the microammeter are secured to, and electrically shortcircuited by, a circular metallic plate 67 having an upwardly projecting rim 68 adapted to embrace the lower end of cup-like support member 57. A plurality of set screws 69 penetrating transversely through the rim 68 provide means for securing the plate 67 to support member 57 and also provide means whereby the orientation of the restrained pendulum assembly 64 may be adjusted relative to the free, or major, pendulum mass 18a.

Contained within the microammeter case 65 is a permanent horseshoe magnet having the pole pieces 70a and 70b providing a suitable air-gap therebetween. The aforesaid magnet is conventionally spaced from the walls of case 65 and affixed relative thereto by means, such as screws and spacers 71. Pivotally supported within the air gap of the magnet by upper and lower jewelled bearings 72 is a vertical shaft 73 carrying a small coil of copper wire 74, the terminals of which are electrically connected in conventional manner (by means not shown) to terminal screws 66a and 66b. The jewelled pivot bearings 72 are rigidly held in place relative to the horseshoe magnet and other supporting structure by suitable brackets 75. As may be seen from Figure 9, the pivot bearings 72 need not be placed so that the shaft 73 is coaxial with respect to the central axis 14. The axis of shaft 73 may, if desired, be eccentric with respect to central longitudinal axis 14 but it is essential that, when the axis of shaft '73 is eccentric to axis 14, then the axis of shaft 73 must be substantially parallel to axis 14.

Encircling the upper and lower ends of shaft 73 are,

'7 respectively, spiral hair springs 76:; and 76b. The inner ends "of spiral springs 76a and 76b may be secured to shaft 73 while the outer ends thereof are secured to support brackets '75. In a preferred form of the invention the Springs 7661 and 76b encircle the shaft 73 in opposite directions, i. e. in a manner such that, when shaft 73 turns in a given direction, it will tend to tighten the spiral 7 6d and simultaneously loosen the spiral 7 6b.

A'ffixed to the lower portion 7 of vertical shaft 73 is a horizontal l'ever arm 77 Which may be the conventional indicator needle of the microammeter mentioned herein'before. Adjacent the outer extremity of thelevcr arm 77 there is secured thereon a minor mass 78. In the preferred embodiment wherein restrained pendulum 64 comprises a direct current microarhmeter, the mass may consist of a small strip of mirror-backed glass whieh is e'emented to the needle or arm 77. The purpose of the mirrored surface will be explained hereinafter.

It will be understood from consideration if the foregoing description that the tension of the two counterwound spiral springs 76a and 76b may be adjusted relative to each other such that, when shaft 73 and central axis 14 are vertical, the arm 77 and minor mass 78 may be caused to take a normal resting position such as is represented by the full-line outline of these elements shown in Figure 9. The position of this normal resting position relative to the center of gravity of the mass 18a may then be adjusted by loosening screws 69 and turning the entire restrained pendulum assembly 64 relative to major mass 18a until minor mass 78 is positioned arcuu ately around axis 14 relative to mass 18a as represented in Figure 9. It will be appreciated that, when the axis 14 is tilted so that its upper end is above the center of gravity CG of the major mass 18a, the arm 77 and the minor mass 78 will then move (against the restraining action of at least one of the spiral springs 76a or 7612) toward the position represented in Figure 9 by the broken line outline of these arm and minor mass elements.

Attention is directed to the fact that the electrically short-circuited coil 74, positioned in the magnetic field existing between the permanent magnet poles 70a and 70b, exerts a damping action (as distinguished from a constant restraining action) which minimizes oscillatory swing of the minor mass 78 around the axis of the shaft 73.- It will be apparent that the coil 74 may, if desired, be replaced by an equivalent small sheet of non-magnetic metal similarly mounted upon the shaft 73. According- 1y, the coil (or sheet of non-magnetic metal) 74 and the permanent magnet having poles 70a and 7011 may be described as magnetic means operatively associated with the I lever arm 77 and the supporting member for damping 's: cillation of the minor mass '78.

The face of the microamme'ter which constitutes a major portion of the structure of restrained pendulum 64 is preferably covered by a sheet of transparent material 79, such asglass or plastic. One or more narrower strips of mirror surfaced glass 80 may be cemented to the transparent material 79 at a predetermined position thereon to provide a reference position indicant similar to the indicant referred to in connection with Figures 1 and 3'.

Positioned within housing 10 below the transparent face of restrained pendulum assembly 64 is a source of light and a photosensitive cell 37 which may be mounted for rotation in fixed relation around the central axis 14-. Since the structure, function, and operation of these elements may be identical with that already described in conjunction with Figures 1 and 4, a repetition of that description here is not necessary.

It is to be understood that the specific forms of apparatus illustrated in the drawings are by way of example and that the invention is not restricted thereto, but that obvious variations thereof may be made by persons skilled in this particular art without departing from the scope of the invention which is to be limited only by the following claims;

What we claim is:

1. An in'clino'rn'eter comprising, in combination: a support member; means mounting said support member for free rotation around the central longitudinal axis of said member; a major mass affixed to said support member with the center of gravity of said mass eccentric to said central longitudinal axis; a weighted arm assembly, said assembly comprising a lever arm movable in a plane substantially perpendicular to said central longitudinal axis, pivot means mounting said le'ver arm upon said support member for rotation of the lever arm around a second axis which is substantially perpendicular to said lever arm plane, a minor mass mounted upon said lever arm with the center of gravity of said minor mass eccentric to said second axis, and resilient spring means anchored at one end thereof to said support member and operatively connected at the other end thereof to said lever arm to restrain rotary motion of the lever arm around said second axis; and means on said support member for adjusting the position of said weighted arm assembly so that the center of gravity of said minor mass is spaced arcuately around said central longitudinal axis relative to the center of gravity 'of said major mass.

2. Apparatus in accordance with claim 1 in which said second axis is coextensive with respect to said central longitudinal axis.

3. Apparatus in accordance with claim 1 in which said pivot means includes a shaft carrying said lever arm, and said resilient spring means consists of a pair of spiral springs encircling said shaft in opposite directions, with the inner end of each spring affixed to said shaft and the outer end of each spring anchored to said support memher.

4. An inclinometer comprising, in combination: a housing; an elongated support member; means mounting said support member within said housing for free rotation around the central longitudinal axis of said support member; a major mass afiixed to said support member with the center of gravity of said mass eccentric to said longitudinal axis; a weighted arm assembly, said assembly comprising a lever arm movable in a plane substantially perpendicular to 'said longitudinal axis, pivot means mounting said lever arm upon said support member for rotation of the lever arm around a second axis which is substantially perpendicular to said lever arm plane, a minor mass mounted upon said lever arm with the center of gravity of said minor mass eccentric to said second axis, and resilient spring means anchored at one end thereof to said support member and operatively connected at the other end thereof to said lever arm to yieldingly restrain rotary motion of said lever arm around said second axis; and means on said support member for initially adjusting the position of said weighted arm assembly so that the center of gravity of said minor mass is spaced arcuately around said central axis a selected amourif relative to the center of gravity of said major mass.

5. An incl'ino'rneter in accofc'l'ance with claim 4 having, in combination therewith: means operatively associated with said support member" and with said housing for damping oscillation of said major mass around said central longitudinal axis whenever said axis is tilted away from vertical.

6. Ari inclihometer in accordance with claim 5 in which said means for damping oscillationo f said major mass includes a viscous fluid, and which said weighted arm assembly includes magnetic means operatively associated with said lever arm and said support member for damping oscillation of said minor mess around said second axis whenever the last-mentioned axis is tilted away from vertical. ,7

7. An inclinometer in accordance with claim 4 and including: a first indica'rit affixed to said' lever arm; a second indicant afiixed to said support member at a selected reference position; and meaiis carriedwi'thin said housing and operatively associated with said' indieants for producing outside said housing indications characteristic of movement of the first indicant relative to the second indicant.

8. An inclinometer in accordance with claim 7 in which both said indicants are plane-mirrored surfaces arranged in planes which are perpendicular to said central longitudinal axis and said means for producing an indication outside said housing includes a light source adapted to revolve around said central longitudinal axis, a photosensitive cell adapted to revolve in fixed relation to said light source and so positioned as to receive reflected light from each of said mirrored surfaces during some portion of each revolution around said longitudinal axis, and means for revolving said light source and said photosensitive cell around said axis at a uniform rate.

References Cited in the file of this patent UNITED STATES PATENTS

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