454 Analyses of Books. {Dec. 



After this statement he proceeds to explain and exemplify the 

 application of the theory to the investigation detailed in the 

 paper ; and then, describing the compass and magnets made 

 use of (the verbal description being illustrated by an engraving), 

 he gives the subjoined account of the mode of experimenting 

 adopted. 



" A meridian line being drawnon a firm table, standing on 

 a stone floor, the compass was accurately adjusted on it, so 

 that the needle pointed to zero on the graduated circle. The 

 magnets were fixed at the bottoms of earthen pans, secured in 

 such a way to rectangular pieces of board that their positions 

 could not be accidentally changed, and projecting from these 

 boards were small pieces of brass, on each of which a line was 

 drawn to indicate the position of the axis of the magnet ; the 

 horizontal distance of the edge of each of the projections nearest 

 to the needle from the corresponding end of the magnet within 

 the pan, was exactly three inches ; 1 could therefore, in any 

 instance, determine very accurately the distance of the centre of 

 the magnet from that of the needle. The pans were placed on 

 the table, so that the indexes on the pieces of brass coincided 

 with the meridian line. Water was now poured into the pans, 

 and the temperature of the magnets was varied by varying the 

 temperature of the water. The temperature of each magnet was 

 ascertained by a thermometer placed in the water, with its bulb 

 resting on that pole of the magnet which was nearest to the 

 centre of the needle. In my first observations I however made 

 use of only one thermometer, which was moved, during them, 



from one magnet to the other.'* 



# * # * # * * 



'' The observations contamed in the tables were made thus : 

 I first noted the time and 



then the temperature of the north magnet ; after which I placed 

 the thermometer on the pole of the south magnet; 1 next 

 observed the westerly point, at which the needle was held in 

 equilibrio by the terrestrial forces and those of the magnets, 

 slightly agitating the needle, that it might the more readily 

 assume the true position ; from this it was led, by means of a 

 very small and weak magnet, held on the outside of the com- 

 pass-box, towards the easterly point of equilibrium, which was 

 observed in the same manner ; and from this it was led in the 



same way towards the southerly point After these 



observations of the points of equilibrium, the temperature of the 



south magnet being observed, the time at 



which the observations concluded, was noted. The temperature 

 of the water in the pans was now increased or diminished, 

 according to circumstances, by the addition of other water, and 

 the pans covered over, to prevent any rapid changes of temper- 

 ature during the observations : after allowing a short time for 



