11 



chronosoopio instruments in the Royal Observatory at Greenwich, it 

 may yet be destined to have large practical applications in engi- 

 neering. 



The process of anastatic printing, which was also the joint inven- 

 tion of William and his brother Werner, belongs to this period. As 

 an evidence of the importance which was early accorded to this 

 invention, it may be stated that it was considered worthy of being 

 made the subject of a Friday evening lecture by Faraday at the 

 Royal Institution in 1845. Siemens himself, in a lecture delivered 

 by him at Birmingham on 20th October, 1881, referring to this in- 

 vention, states that the favourable notice taken of it by Faraday 

 " obtained for me an entry into scientific circles, and helped to sustain 

 me in difficulty." 



Another of William Siemens' early inventions was his water-meter, 

 which exactly met an important practical requirement, and has had 

 a splendid thirty years' success. It applied curiously subtle hydraulic 

 principles, which, even irrespectively of the practical value of the 

 instrument, are of great interest. Imagine a Barker's mill running 

 absolutely unresisted. The discharged water must have approximately 

 zero absolute velocity on leaving the nozzles ; in other words, its 

 velocity relatively to the nozzles must be approximately equal to the 

 contrary absolute velocity of the nozzles. Hence the machine will 

 rotate ill simple proportion to the quantity of water passing through 

 it. By an extension of similar considerations, it is easy to prove that 

 if the wheel, instead of being unresisted, is resisted by a force exactly 

 proportional to the square of its angular velocity, its velocity must 

 still be proportional to the quantity of water passing through it per 

 unit of time. Thus, provided this law of resistance is maintained, 

 the whole angle turned through by the wheel measures the whole 

 quantity of water that has passed. Now think of the difficulties 

 which Siemens had to overcome to apply this principle. What has 

 been roughly called a Barker's mill, must be completely inclosed in 

 the supply water-pipes, its nozzles discharging into water, not into 

 air. It must be of very small dimensions to be convenient for practice, 

 and its bearings must be kept oiled to secure, not only that it may not 

 be injured by the wear of running for years, but also that the con- 

 stant frictional force of solid rubbing on solid may be as nothing 

 compared to the resistance, proportional to the square of the velocity, 

 exerted by the circumambient liquid upon a wheel with sharp edged 

 vanes rotating in it. After a few years of trials, difficulty after 

 difficulty was overcome, and the instrument did its work with the 

 accuracy and convenience which met practical requirements. It was 

 we believe the protection offered by the British Patent Law which, 

 in the case of this very instrument, allowed Siemens to work it out 

 in England, and so helped him eventually to find his home among us, 



