78 



MAGNETISM. 



when let down on the planes, there will 

 frequently be as great an error in the 

 dip. It is true that the part of the agate 

 planes on which the axis rests when 

 the vibrations are stopped, will be a 

 little different, according to the point 

 at which the needle stood before it was 

 let down ; which will make a small dif- 

 ference in the dip, as shown by the di- 

 vided circles, when only one end of the 

 needle is observed, though the real dip, 

 or inclination of the needle to the hori- 

 zon is not altered ; but this difference 

 is far too small to be perceptible ; so 

 that the above-mentioned error cannot 

 be owing ' t to this cause. Neither does 

 it seem to arise from any irregularity 

 in the surface of the agate planes, for 

 they were ground and polished with 

 great accuracy ; but it most likely pro- 

 ceeds from the axis slipping in the 

 large vibrations, so as to make the 

 agate planes bear against a different part 

 of it from what they would otherwise 

 dp. Mr. Cavendish gives it as his opi- 

 nion that this irregularity is not owing 

 either to want of care or skill in the 

 execution, but to the unavoidable im- 

 perfection of this kind, of work. He 

 imagines, therefore, that this instrument 

 is at least as exact, if not more so, 

 than any which has yet been made. 



(300.) Thus it appears, that in general 

 the indications afforded by the dipping- 

 needle are liable to two principal sources 

 of error : first, the axis of the magnet's 

 length may not be the exact axis of its 

 magnetic forces ; and, secondly, its point 

 of suspension may not, in every position, 

 be exactly coincident with its centre of 

 gravity. Different modes of observa- 

 tion must be resorted to, in order to 

 ascertain the amount of the errors aris- 

 ing from these causes. We must first 

 assure ourselves that the axis of rota- 

 tion of the needle is perfectly level, so 

 that the needle shall turn in a plane ex- 

 actly vertical : we must next see that it 

 is placed accurately in the plane of the 

 magnetic meridian ; and we have then 

 to observe carefully the positions at 

 which it settles, after being repeatedly 

 disturbed, and allowed to oscillate freely. 

 The mean of these positions may then 

 be taken as the true position of the 

 needle under these circumstances. We 

 are next to turn the whole instrument 

 horizontally till it has described a com- 

 plete semicircle, or 180; that face of it 

 which was to the east being now to the 

 west, and vice versa: and then, taking- 

 similar observations on the dip, we get 



a mean of these, for this new position. 

 Comparing these two means, we obtain 

 a resulting mean, which is free from 

 the first source of error. In order to 

 exclude the operation of the second cause 

 of error, we must now remove the 

 needle from its supports, and after de- 

 stroying its magnetism, magnetize it 

 again in the contrary sense; namely, 

 rendering that end a north pole, which 

 before was south, and vice versa; 

 then, replacing it upon its supports, we 

 must make with it similar sets of ob- 

 servations to those made before, turning 

 it first on one side and then on the other. 

 The mean thus obtained, combined with 

 the (former mean, will give the mean of 

 the whole ; which may be considered as 

 the true dip, at the place and at the 

 time of observation, 



(301.) The error produced by the 

 want of coincidence between the axis of 

 motion and the centre of gravity of the 

 needle may be removed by the following 

 method, devised by Daniel Bernouilli, 

 and which, being easily executed, de- 

 serves to be generally known. Let a 

 dipping-needle be constructed with as 

 much correctness as can be effected by 

 the ordinary methods of workmanship, 

 and balanced as exactly as possible be- 

 fore it is rendered magnetic : when im- 

 pregnated, therefore, it will arrange it- 

 self tolerably nearly in the line of dip. 

 Carefully note the position it takes un- 

 der these circumstances, and then de- 

 stroy its magnetism. When it has thus 

 returned to its natural state, alter the 

 point of suspension, or adjust the centre 

 of gravity, in such a manner as that it 

 shall arrange itself in the same position 

 as that above noted by the sole in- 

 fluence of gravity. Now impregnate it 

 again, imparting to it the same poles as 

 before. It is evident that it will now ap- 

 proximate still more nearly to the true 

 line of the dip, since nearly the whole of 

 that portion of the force of gravity 

 which before produced a deviation from 

 the position no longer operates. If we 

 find that this approximated position dif- 

 fers several degrees from the former one, 

 the operation may be repeated, until we 

 have arrived so near to the true position, 

 that no further difference can be per- 

 ceived. It will rarely happen that the 

 third approximation will give an error 

 of half a degree. 



(302.) This simple instrument was 

 adapted by its author to observation in 

 all situations, in the following ingenious 

 manner: A very light brass graduated 



