XX 



means of wires descending vertically in the open tubes, a circuit closed 

 by the contact of the wire with either column of mercury indicating by 

 electro-magnetic action the moment at which the contact takes place, 

 and by inference the position of the point of contact. Precisely the same 

 principle, with minor modifications, has of late years been practically ap- 

 plied by Padre Secchi, and more recently by M. van Eysselberghe. 



Another is the electro-magnetic counter, by which the number of any 

 given repeated mechanical actions is readily and certainly recorded ; and 

 the electric chronoscope, by which very small intervals of time may be 

 determined (as, for example, in estimating the velocity of projectiles), 

 must also be mentioned. 



Cryptography, or the art of writing in ciphers, is indebted to him for 

 a compendious apparatus, by which the characters representing the suc- 

 cessive letters of the written communication are periodically changed by 

 the machine, so as to defy the ordinary well-known methods of decipher- 

 ing. The receiver, possessing a machiue set by agreement in the same 

 manner as that of the sender, can immediately decipher the dispatch by 

 reversing the process by which the cipher was constructed. 



The fact, discovered by Brewster, that the plane of polarization of the 

 sky is always 90° from the sun, gave rise to Wheatstone's " Polar Clock," 

 which consists of a double-image prism and a thin plate of selenite en- 

 closed in a tube placed parallel to the earth's axis. When the prism, 

 which carries an index traversing a circular arc marked with the hours, 

 is turned round until no colour is perceived, the index, when once ad- 

 justed, will mark the hour of the day ; this is a pleasing philosophical 

 toy, but of little practical utility. 



The name of Wheatstone is intimately connected with the earliest 

 development of spectrum-analysis. In a paper on " The Prismatic Ana- 

 lysis of Electric Light," read before the Meeting of the British Associa- 

 tion at Dublin in 1835, he announced the existence of bright lines in the 

 spectrum, emitted by the incandescent vapour of metals volatilized by the 

 heat of an electric discharge. He was then the first to demonstrate the 

 fact that the spectrum of the electric spark from different metals pre- 

 sented respectively more or less numerous rays of definite refrangibility, 

 producing each a series of lines differing in position and colour from each 

 other, and that thus the presence of a very minute portion of any given 

 metal might be determined. " We have here," he adds, " a mode of dis- 

 criminating metallic bodies more readily than that of chemical examination, 

 and which may hereafter be employed for useful purposes." This remark 

 has been abundantly verified by the many important recent investigations 

 that have been based on specLrum-analysis — such, for example, as the dis- 

 covery and isolation of thallium by Mr. Crookes. 



It would occupy too much space to enumerate all Wheatstone's inge- 

 nious contrivances ; but his most important contribution to the wants or 

 conveniences of civilization, the practical electric telegraph, must now be 



