MAGNETISM. 



79 



circle, ABC,/"-. 69, is fixed to one side as possible before impregnation. A very 

 of the needle SN, concentric with its light index, Rr, is then fitted on the axis 

 axis, and the whole is balanced as nicely so as to turn stiffly upon it. This will 



Fig. 69. 



destroy the equilibrium of the needle. 

 If the needle had been made with per- 

 fect accuracy, and perfectly balanced, 

 the addition of this index would cause 

 it always to settle with the index perpen- 

 dicular to the horizon, whatever degree 

 of the circle it might chance to point 

 at. But as this is not to be expected, 

 the index is to be set at various degrees 

 of the circle, and the position which the 

 unmagnetic needle takes, corresponding 

 to each place of the index, must be 

 observed, and the result of all these 

 observations recorded in a table. Sup- 

 pose, for example, that when the index 

 is at 50, the needle inclines 46 from 

 the horizon : if ;in any place we ob- 

 serve that the needle, rendered magnetic 

 by juxtaposition between two powerful 

 magnets, having the index at 50, has 

 an inclination at 46, we may be certain 

 that this is the true dip at that place ; for 

 the needle is not deranged by magne- 

 tism from the position which gravity 

 alone would give it. As we generally 

 know something of the dip that is to 

 be expected in any place, we must set 

 the index accordingly. If the needle do 

 not show the expected dip, the position 

 of the index must be altered, and the in- 

 clination of the needle again observed. 

 Examine whether this second position 

 of the index, and this dip, form a pair 

 which is in the table : if they do, then 

 we have obtained the true dip; if not, 

 we must try another position of the 

 index. Noticing whether the agree- 

 ment cf this last be greater or less than 

 those of the former pair, we learn 

 whether to change the position of the 

 index in the same direction as before, or 

 in the opposite direction. Professor 

 Robison had a dipping-needle of this 

 kind, made by a person totally unac- 

 quainted with the making of philoso- 

 phical instruments. He used it at 

 Leith, at Cronstadt in Russia, at Scar- 

 borough, and at New York, and the 

 dip indicated by it did not in any single 

 trial differ a degree and a half from 



other trials, or from the dip observed 

 by the finest instruments. He tried it 

 in Leith Roads in a rough sea; and 

 did not think it inferior, either in cer- 

 tainty or dispatch, to a needle of the 

 most elaborate construction. Professor 

 Robison deems it worthy of its inge- 

 nious author, and of the public notice, 

 because it can be made for a moderate 

 expense, and, therefore, may be the 

 means of multiplying observations on 

 the dip, which are of immense value to- 

 wards perfecting the theory of terrestrial 

 magnetism. 



(303.) In a dipping-needle recently 

 made by M. Gambey, at Paris, intended 

 to be used at St. Petersburgh, the axis, 

 instead of being a cylinder, is a knife- 

 edge, as in a fine hydrostatic balance. 

 This edge is placed exactly in the 

 centre of gravity of the whole com- 

 pound needle, and is so fixed that when 

 the needle dips 71, the edge rests per- 

 pendicularly on two agate plates. It is 

 evident that such a needle, however sen- 

 sible, is adapted for use only in those 

 situations in which the dip is nearly 

 71. It is, however, well calculated for 

 ascertaining minute variations of incli- 

 nation in the same place *. 



(304.) Another mode of dispensing 

 with the condition that the axis of mo- 

 tion should accurately pass through the 

 centre of gravity, a condition which it is 

 next to impossible ever strictly to fulfil, 

 is that adopted in the dipping-needle 

 invented by Professor I. Tobias Mayer, 

 in his treatise, De Usu accuratiori 

 acus indinatorice Magneticce t. 



The centres of motion and of gravity 

 are, in this needle, designedly separated, 

 so that the inequalities of workmanship 

 in the axis, or in the planes of suspen- 

 sion, are rendered of less effect, being 

 opposed by thejoint influence of gravity 

 and magnetism; whilst, by a peculiar 

 process of observation, and an appro- 



* Annales de Chimie etde Physique, 

 f Published in the Transactions of the Rojal So. 

 ciety of Sciences atGottingen, for 18 J 4. 



