342 FRAGMENTS OF SCIENCE 



tion double that of the latter, a very slight motion of 

 the magnet is sufficient to produce a displacement of the 

 image through several yards. 



This then is the principle of the beautiful apparatus 1 

 by which the investigation was conducted. It is manifest 

 that if a second helix be placed between the poles s N 

 with a cylinder within it, the action upon the astatic mag- 

 net may be exalted. This was the arrangement made use 

 of in the actual inquiry. Thus to intensify the feeble ac- 

 tion, which it is here our object to seek, we have in the 

 first place neutralized the action of the earth upon the 

 magnets, by placing them astatically. Secondly, by mak- 

 ing use of two cylinders, and permitting them to act 

 simultaneously on the four poles of the magnets, we have 

 rendered the deflecting force four times what it would be, 

 if only a single pole were used. Finally, the whole ap- 

 paratus was enclosed in a suitable case which protected 

 the magnets from air-currents, and the deflections were 

 read off through a glass plate in the case, by means of 

 a telescope and scale placed at a considerable distance 

 from the instrument. 



A pair of bismuth cylinders was first examined. Send- 

 ing a current through the helices, and observing that the 

 magnets swung perfectly free, it was first arranged that 

 the bismuth cylinders within the helices had their central 

 or neutral points opposite to the poles of the magnets. 

 All being at rest, the number on the scale marked by the 

 cross wire of the telescope was 572. The cylinders were 

 then moved, one up, the other down, so that two of their 

 ends were brought to bear simultaneously upon the mag- 



1 Devised by Professor "W. Weber, and constructed by M. Leyser, of Leipzig. 



