The Philosophical Instruments in the Paris Exhibition. 431 



of the laws of electro-magnetism, to astronomical purposes. 

 The Americans were. among the first to appiy them to deter- 

 mine the difference of longitude at various places. The most 

 interesting object in this department of electricity was Pro- 

 fessor Bond's astronomical clock and chronograph, which were 

 used for ascertaining, in connection with the Atlantic Telegraph 

 Cable, the difference of longitude between Newfoundland and 

 Valentia. The apparatus of this clock and chronograph are 

 so contrived, that the precise epoch of an observation can be 

 registered to the one-fiftieth part of a second. Such is the 

 wonderful accuracy of observation to which modern science 

 has attained. As appertaining to this apparatus, Sir W. 

 Thomson's ingenious electro-dynamometer may be noticed, 

 which was exhibited by Messrs. Elliott. Sir W. Thomson has 

 been long engaged in perfecting this instrument, which was of 

 signal use in laying the Atlantic Telegraph Cable; and he 

 has lately constructed one of such delicacy, as to be capable of 

 measuring differences of potential, ranging from one-four- 

 hundredth of a DanielPs cell up to 100,000 cells. 



The best chronoscopes and chronographs were exhibited 

 by France. The principal use of these instruments is to 

 determine the velocity of projectiles by electricity, and to 

 register the precise time at which an astronomical observation 

 is made. Those constructed by B. Hardy for the French 

 Government are admirable specimens of mechanical ingenuity. 



A very clever contrivance for engraving by electricity was 

 exhibited in the machinery department. A metal plate, having 

 the design which is to be engraved drawn on it with a 

 particular kind of ink, is slowly rotated, while several other 

 plates, on which the design is to be engraved, are also slowly 

 rotated. The engraving is then effected by applying a 

 diamond cutting-point to the face of each plate, which is 

 pressed against it, through the agency of an electrical current, 

 whenever a blunt point presented to the first plate encounters 

 the ink, but is withdrawn at other times. The point presented 

 to the first plate is a feeler which determines by electrical 

 agency whether there is ink beneath it or not. If there is, 

 the diamond points opposite to all the other plates are pressed 

 in ; if there is not, they are withdrawn, and do not act. 



A new and powerful electrical machine was exhibited by 

 P. Topler, of Riga. It acts on the principle of multiplying 

 induction resulting from a series of glass parallel discs rotating 

 rapidly. The sparks produced by this machine are very 

 numerous and powerful. France exhibited a great variety of 

 magnificent magneto-electric apparatus, constructed for the 

 Ecole Polytechnique by Nollett and RuhmkorfT. The best 

 application of electricity for the production of electric light 



