NATURAL PHILOSOPHY. 163 



corresponds to the doctrine, now generally admitted as estab- 

 lished, of the correlation of the physical forces. It is also, in 

 form, more closely analogous to the law expressive of the mutual 

 action of material masses under the influence of gravity. 



In the case of gravity, the action was proportioned dii-ectly to 

 _ the product of the masses, and inversely to the square of their 

 'distance. _ In the case of electrical currents, the direction of the 

 elements is to be taken into consideration ; and the resulting force 

 may be expressed as the product of the quotients of each element 

 (considered both as to strength of current and length and direc- 

 tion of the element), divided by their distance (also considered 

 both as to length and direction). The consideration of direction 

 involves the application of the newly-developed principles of the 

 mathematical science of quarternions. 



The law above mentioned being assumed, all of the other ordi- 

 nary special laws of electrical action flow readily and necessarily 

 therefrom ; as, for example, the mutual action of closed circuits, 

 and the action of magnets considered as solenoids. — E. B. 

 Elliot, in Froc. Am. Ass'n,for Adv. of Science, 1866. 



INSTRUMENT FOK SHOWING MINUTE CHANGES OP MAGNETIC 



DECLINATION. 



Dr. Joule described, before the Literaiy and Philosophical Soci- 

 ety, an instrument he had constructed for rapidly showing minute 

 changes of magnetic declination. A column of small magnetic 

 needles is suspended by a filament of silk. Attached to the lower 

 end of the column is a glass lever with a hook at its end. A 

 second fine bent glass lever is suspended by another filament of 

 silk ; its shorter arm l^eing connected with the first lever by means 

 of the small hook. The whole is enclosed in a stout copper box. 

 Light is admitted into the box through a lens, cemented into an 

 orifice immediately under the object-glass of a microscope, placed 

 over the free extremity of the bent lever. The microscope mag- 

 nifies about three hundred linear, and has a micrometer in its eye- 

 piece, with divisions corresponding to one two-thousandth of an inch. 

 One division corresponds to a deflection of the needle of four and 

 one-half minutes, and, as a tenth of a division can be vei-y readily 

 observed, the instrument measures deflections to within half a 

 second. So rapid is the action, that, on aj^plying a small mag- 

 netic force, the index takes up its new position steadily in two 

 seconds of time. Besides being a damper to the motion of the 

 needle, the copper box, by its conducting power, equalizes the 

 temperature rapidly, so that the indications are not to any consid- 

 erable extent disturbed by currents of air. The success of the 

 present instrument encourages the hope that very much greater 

 delicacy may yet be obtained. Dr. Joule said that he had ob- 

 served an extensive magnetic disturbance the preceding evening, 

 the index being driven entirely out of the field of view. 



