MAGNETISM. 



621 



series of magnetic observations from the year 1813 to 

 1821 j during which period the needle passed through 

 its maximum of westerly variation in London. 



The principal part of this instrument, viz., its 

 needle, with the box and divided arc, to measure 

 the angle of variation, is very similar to the instru- 

 ment in the possession of the Royal Society ; but it is 

 greatly improved by the addition of a small transit 

 telescope, which readily and accurately determines 

 the true meridian, or the zero from which the de- 

 grees of the variation is to be measured. The box 

 containing the needle is not fixed, as in many com- 

 passes, but turns horizontally on the centre, and has 

 an index fastened to it, pointing to a divided arc on 

 the brass frame on which it turns. The method of 

 observing is, to move the box until a line drawn on 

 it points exactly to the end of the needle, which 

 being done, the angle of variation is shown by the 

 divisions on the arc ; F F is a mahogany board, which 

 is the support of the whole instrument ; it stands on 

 the points of three screws G, H, I. by which it can 

 be levelled. Above this is a flat plate of brass E E, 

 attached to the board by the centre pin, and resting 

 upon three studs projecting from the board, to insure 

 its having a perfect bearing, whilst it admits of a 

 small horizontal motion round the centre by means 

 of the screw VV X. The plate E E has the divided 

 arc wt, fixed upon one end of it ; and a vernier, D, 

 traverses against the divisions ; it is divided upon a 

 projecting part of a brass plate C C, which moves 

 upon the common centre pin of the plate E, and also 

 the needle A A. The plate C C has two segments 

 of brass at each end of the needle, and these have 

 the centre lines drawn upon them, which are brought 

 to the points of the needle when the observation is 

 made. A light brass box d d, with a glass cover, is 

 fitted over the plate C C, to preserve the needle 

 from disturbance by the wind ; it also supports a 

 small double microscope M, intended to assist in 

 examining when the index line points exactly to the 

 end of the needle ; the stem of the microscope is 

 fitted to a dovetail groove, and can be removed to 

 the opposite end of the box at pleasure. The centre 

 pin of the plates E and C terminate in an extremely 

 fine point, on which the needle is suspended, having 

 an agate cap, I >, to diminish the friction as much as 

 possible. The needle is of a cylindrical figure, about 

 the five-hundredth part of an inch in diameter, and 

 ten inches long ; weighs sixty-five grains and a half, 

 and is terminated by two conical points ; the circular 

 enlargement of the centre has a hole through it, for 

 the reception of the brass socket B, which has the 

 agate cap fixed in it ; the needle is, of course, pro- 

 vided with the usual apparatus for lifting it off the 

 point when not in use, to avoid wearing the point of 

 suspension ; a, a, are two arms, screwing down upon 

 the plate, carrying the vernier D ; they support a 

 brass frame, which has the usual clamp screw L, and 

 tangent screw II, the former to fasten it to the arc 

 JM, and the latter to give a slow motion, and adjust 

 the box for the observation. The transit telescope, 

 O P, is supported over the instrument by two pillars, 

 N, N, fixed on the brass plate, E E, and having small 

 frames or boxes/, at the top, for the reception of the 

 y's, in which the pivots of the axis Q, of the tele- 

 scope are supported : this axis is conical, and is fixed 

 exactly at right angles to the tube O P, in the same 

 manner as other transit instruments. On the extre- 

 mity of one of the pivots of the axis, a small divided 

 circle H, is fixed, and has an arm at the opposite side 

 turning about at the centre, and provided with ver- 

 niers to read the divisions of the circle ; to this arm 

 a small level S, is attached ; the whole forming a 

 vertical index to set the telescope at any required 

 altitude, by setting the index at the proper division, 



and then moving the telescope till the bubble of the 

 level, S, shows the index to be horizontal ; the eye- 

 piece, A, of the telescope has a small dovetail in it to 

 admit a dark glass for observation of the sun ; g g. 

 are the screws for adjusting the wires in the eye- 

 piece as usual ; there is also a detached level to 

 adjust the instrument. 



First, place the feet of the detached level upon 

 the brass plate E E, in different directions, and bring 

 it level by the screws H, I, G ; then apply the level 

 upon the two pivots of the transit, the covers of the 

 boxes/ opening with hinges for this purpose, and 

 the bottom of the feet having notches to rest upon 

 the pivots ; if the axis does not prove level, one of 

 the y's must be elevated or depressed by a screw in 

 the farther pillar N, until the level stands hori- 

 zontally, and, reversing it, end for end, proves every 

 thing to be correct. 



Now set the vernier D, at zero, and put an addi- 

 tional object glass over the glass, P, of the telescope, 

 and in this state the marks A A, against which the 

 needle reads, can be seen through it when directed 

 to them ; this proves the zero of the division to be 

 exactly in the plane of the telescope's motion ; a 

 small screw near Q will rectify it, if necessary, by 

 moving the y a small quantity in the box /; the 

 transit may be reversed in its y's to rectify every 

 thing. 



The telescope is now to be adjusted to the meri- 

 dian by the transit of the stars in the usual manner, 

 the screws W, X, being used to turn the whole 

 instrument round when requisite ; but after these 

 observations have been once made in a satisfactory 

 manner, distant marks should be set up, both nortli 

 and south, and these will give the means of adjust- 

 ing it at once, and in the daytime. 



The needle being suffered to settle, the box is 

 turned about on its centre till its mark comes very 

 near the point of the needle ; the clamp screw, L, 

 is then fastened, and the screw, R, is employed to 

 make the coincidence exact, in which the microscope, 

 M, greatly assists the eye. The vernier, D, now 

 shows the angle of variation. It is proper, after the 

 needle has been once observed, to attract it with a 

 piece of iron, and cause it to make a slight vibration. 

 When it settles again, it will not in all cases come 

 to the same point, because of the friction of the sus- 

 pending point ; it is therefore advisable to make this 

 trial three or four times, and take the mean of the 

 whole. 



The instrument is placed on a pillar in the open 

 air, free from any iron, and is adjusted before every 

 observation ; the vernier is divided into half minutes, 

 and if the meridian be not at a considerable dis- 

 tance, an advantage will be gained by diminish, 

 ing the aperture of the object glass by a pasteboard 

 cape. See Variation. 



It may be worth while to remark, that this instru- 

 ment will serve as a transit instrument for naval 

 officers to examine the rate of their chronometers 

 while on shore. 



The only other form of the magnetic needle, which 

 we shall here describe, is the dipping needle. This 

 needle is so fixed, that instead of playing horizontally, 

 its axis is parallel to the horizon ; and therefore the 

 only motion of the needle must be in a vertical direc- 

 tion. The extremities of the needle traverse a circu- 

 lar ring,divided into degrees and minutes, and adjusted 

 by a spirit level, or otherwise, so that the zero shall 

 stand at the end of the needle, when it,is horizontal. 

 The inventor of the dipping needle was Robert Nor- 

 man, a compass-maker at Ratclifl'e, about the year 

 1580. Some have endeavoured to find the latitude 

 and longitude of places by means of the dipping 

 needle. 



