224 The Astronomer Royal's Experiments on 



axis transversal to the direction which the compass-needle had 

 taken before the constant magnet was introduced, and to 

 observe the deflection produced. The measure of the force of 

 the large magnet was then the cotangent of the angle of devia- 

 tion. The observation of the deflected needle by dots &c. was 

 the same as before; but the angle of deviation was never 

 measured by degrees. Instead of that measure, a circle upon 

 semitransparent paper was graduated by cotangents, and thus 

 the measure of the force of the large magnet was read off at 

 once. 



The arrangements in this state were confided to Mr. Carpenter, 

 Assistant, of the Royal Observatory, by whom all the subsequent 

 arrangements were planned and all the observations were made. 

 I need not say that they were made with the utmost skill and 

 delicacy. 



A small frame was constructed, carrying a floor at a definite 

 position about 1*8 inch above the compass-needle. As it was 

 my object to make the observations at small distances from the 

 great magnet, where its power is great, it was necessary to use 

 a large power in the deflecting-magnet. Mr. Carpenter selected 

 a horse-shoe magnet about 4 inches long, consisting of sixteen 

 plates, each O06 inch thick ; these were retouched a few days 

 before they were used. Prom the consistency of the results 

 obtained at the beginning and end of each circuit of the great 

 magnet, I am entitled to conclude that no sensible change took 

 place in the magnetism of the horse-shoe magnet. The magnet 

 was placed in a vertical position, its two poles resting on the 

 raised floor above mentioned. In all cases the deflecting-magnet 

 was used in the two positions to produce deflection right and 

 deflection left. 



These arrangements sufficed for observation of the powers of 

 the great magnet in both positions, and also for observation of 

 the galvanic coil carrying the soft-iron core, the intensity of the 

 battery having been in some measure adjusted to make the 

 power of the coil with core not very different from that of the 

 magnet. But when the coil was used without core, the force 

 was so enormously reduced that the arrangement which applied 

 well in the other cases failed totally in this. A small magnet 

 was then used, 1*25 inch long, not very strongly magnetized ; 

 its deflecting-power was compared with that of the horse-shoe 

 magnet in the following way : — The small compass being under 

 the influence of the earth's magnetism, the horse-shoe magnet 

 and the small magnet were successively placed on the raised 

 floor above mentioned, then 0*5 inch higher, then 1*0 inch 

 higher; and the cotangents of deflection were compared. Thus 

 the following proportions were obtained : — 



