On a Simple Variation Compass. By WILLIAM SWAN, F.R.S.E., F.R.S.S.A.* About two years ago, my friend Mr John Adie communicated to the Royal Scottish Society of Arts the description of a new variation compass. His instrument, which is intended to be used along with an ordinary theodolite, was devised for the purpose of ascertaining the magnetic meridian with greater accuracy than is attainable, either by the use of the compass usually attached to theodolites, or by employing the more ordinary forms of the azimuth compass. Mr Adie's very elegant invention is described in the Transactions of the Royal Scottish Society of Arts, vol. iv., p. 138. It consists of a delicately-suspended compass-needle, inclosed in a tube furnished with collars, which are placed in the Ys of the theodolite, the telescope having been previously removed. The ends of the needle, which are brought to fine points, are nearly in contact with finely divided glass diaphragms; and the needle being viewed through the diaphragms by powerful eye-pieces, has its ends accurately referred to those divisions. It is easy to see how, in this manner, the axis of the tube with its collars,-which, when placed in the Ys, is coincident with the axis of the theodolite telescope occupying that situation,-can be placed parallel to the axis of the needle; and the reading on the horizontal limb of the theodolite corresponding to magnetic north may be obtained. From actual trial, I was so much satisfied of the excellence and utility of Mr Adie's instrument, that I felt desirous of having something of the same kind applied to a Kater's altitude and azimuth circle in my possession; but as the telescope of that instrument, unlike that of the ordinary theodolite, does not admit of being removed, I was obliged to adopt an arrangement totally different from Mr Adie's. The instrument I devised was constructed for me by Mr Adie in the autumn of 1852; and I now describe it, in the hope that it may be useful to persons who, possessing instru *Read before the Royal Scottish Society of Arts. ments analogous to Kater's circle or, indeed any form of theodolite, may wish to make observations of magnetic declination. It consists of a collar A, fitted so as to slide without much friction upon the object-end of the telescope of the theodolite with which it is to be used; an arm B, projecting in front of the telescope, furnished with a fine steel point C; and a small collimating magnet LF, supported on an agate cap, which turns on the point C. The best form for the collimating magnet, would, I conceive, be that of a hollow steel cylinder, carrying at one end a lens L, and at the other a cross of spiderlines F, as represented in the figure,-a construction which has been adopted in various magnetic declinometers. In the instrument made for me by Mr Adie, instead of the cylinder shown in the figure, there are two steel plates, each 5 inches long, 0-3 broad, and 0.02 thick, placed parallel to each other, and connected at the ends by light frames of brass; an arrangement which answers exceedingly well. One of these frames carries the lens L, and immediately behind the other, and between the plate, so as to be out of risk of injury, is placed a diaphragm carrying the cross fibres F. The lens is not achromatic, but as its aperture is only 0.2 inch, while its focal length is 4.7 inches, the image of the cross fibres formed by it is tolerably well defined. I should recommend, however, the adoption of an achromatic lens of greater aperture, and shorter focal length than that which I have described, and the hollow cylindrical magnet instead of the parallel steel plates; for the cylindrical magnet will admit of the lens and cross lines of the collimator being more firmly fixed in their places, while at the same time they will be less liable to derangement from handling the magnet. It is scarcely necessary to explain that the rays of light proceeding from the cross fibres, which are placed in the principal focus of the lens are ren dered parallel by the lens, and thus enter the telescope of the theodolite in a fit state to be brought to focus at the diaphragm wires, where they form a distinct image of the cross fibres. A light tube represented in the figure by dotted lines, slides over the whole, so as to protect the magnet from currents of air; and is furnished with an aperture at its end, covered with glass, through which light is thrown by a small reflector to illuminate the cross fibres. The method of observation consists in first making the image of the intersection of the collimator cross fibres coincide with the middle diaphragm wire of the theodolite telescope, which is easily effected by means of the tangent screws of that instrument, and then reading off the verniers on its horizontal limb. If the magnetic axis of the magnet were parallel to the optical axis of the collimator, the reading on the limb for magnetic north or south would thus be at once obtained; but as such a condition can never be strictly fulfilled in practice, it is necessary, where an accurate result is wanted, to repeat the observation with the needle in an inverted position. For that purpose the agate cap is made to screw into opposite sides. of the magnet, which thus admits of being suspended with either side uppermost. By taking the mean of the readings in the two positions of the magnet, any error caused by want of parallelism in the line of collimation and the magnetic axis will be either wholly or nearly eliminated. Half the difference of the readings in the two positions of the needle, carefully determined from a number of observations, may be registered and applied as an index error, when the needle has been observed without having been inverted; and such a mode of observation will probably be sufficiently accurate for the ordinary purposes of the surveyor. It is desirable that the instrument be adjusted so that the difference of the readings in the two positions of the magnet may not be great. For as the correction, for want of perfect adjustment, obtained by taking the mean of those readings, will generally be only approximate, it is well that any residual error should be confined within as narrow limits as possible. In order to ascertain the variation of the compass, or to VOL. IV. Ꮓ apply the observations of the magnet to the ordinary purposes of surveying, it is necessary to direct the theodolite telescope to a meridian mark, or other proper object, and to read off its horizontal limb; and it is desirable that this should be done both before and after observing the magnet. The collar A should be adjusted to the telescope before taking the observations of the meridian mark; and the magnet and its cover should be put in their places, and removed again, with as delicate manipulation as possible, in order to avoid disturbing the theodolite, the cover for that purpose being made to slide off and on with very little friction. Practically, I have found no sensible discrepancy in the readings for the meridian mark arising from disturbances caused by handling the magnet and its cover; but if it be deemed desirable to avoid altogether the chance of such errors, it may be done by furnishing the aperture in the cover which illuminates the collimator cross, with a piece of parallel plate-glass. The meridian mark may then be seen through this glass, and observed without removing the cover, immediately after observing the magnet. Any error due to refraction will be eliminated by reversing the cover, when it is replaced after reversing the magnet, and again observing the meridian mark; but a good piece of glass, such as that which is used for making the mirrors of sextants, will cause no error from refraction appreciable with the magnifying power of an ordinary theodolite telescope. It is always proper, however, to reverse the cover, in order to eliminate the effects of any attraction it may exert on the magnet; and for the same purpose, I have always observed with the vertical limb of my Kater's circle facing alternately east and west. I may add, with reference to the observations of magnetic declination given in the sequel, that I have since ascertained that when that instrument was brought as near as was possible to a collimating magnet, suspended by a very delicate silk thread, and observed through a telescope, it caused no perceptible deflection. The practical limit to the accuracy of observations made by such an instrument as that which I have described, is the friction of the point of suspension. When the needle shows any symptom of not swinging freely, the point should be carefully sharpened on a hone,-a process which any one may learn to perform for himself. I find that the most consistent readings are obtained, not by waiting until the magnet comes to rest, but by causing the theodolite wire to bisect the arc of vibration of the magnet, by estimation, as soon as that arc is reduced to about 8' or 10'. If the magnet has come to rest, it is easy to make it vibrate again in a small arc, by cautiously approaching to it a magnet or a piece of iron, which is again removed to a sufficient distance before making the observation. As an example of the performance of the instrument, I select the last observation of magnetic declination I have made.* Azimuth of the magnetic axis of the magnet = 77° 44′ 42′′ =102 48 44 The Kater's circle, by means of which these observations. were made, has both its vertical and horizontal limbs 6.5 inches in diameter, each furnished with two verniers, reading 10". The azimuth of the true north was deduced from transits of the sun, taken near the meridian, in the following manner :-The vertical circle being placed approximately in This paper was read on 19th June 1854. |