92 RayleigKs Address to the British Association. 



rightly the value, and even the correctness of recent work. It is 

 not always that which seems at first the most important that 

 proves in the end to be so. The history of science teems with 

 examples of discoveries which attracted little notice at the time, 

 but afterwards have taken root downwards and borne much fruit 

 upwards. 



One of the most striking advances of recent years is in the 

 production and application of electricity upon a large scale — a 

 subject to which I have already had occasion to allude in con- 

 nection with the work of Sir W. Siemens. The dynamo machine 

 is indeed founded upon discoveries of Faraday, now more than 

 half a century old ; but it has required the protracted labors of 

 many inventors to bring it to its present high degree of efficiency. 

 Looking back at the matter, it seems strange that progress should 

 have been so slow. I do not refer to details of design, the elabo- 

 ation of which must always, I suppose, require the experience of 

 actual work to indicate what parts are structually weaker than 

 they should be, or are exposed to undue wear and tear. But 

 with regard to the main features of the problem, it would almost 

 seem as if the difficulty lay in want of faith. Long ago it was 

 recognized that electricity derived from chemical action is (on a 

 large scale) too expensive a source of mechanical power, notwith- 

 standing the fact that (as proved by Joule in 1846) the conver- 

 sion of electrical into mechanical work can be effected with 

 great economy. From this it is an evident consequence that 

 electricity may advantageously be obained from mechanical 

 power ; and one cannot help thinking that if the fact had been 

 borne steadily in mind, the development of the dynamo might 

 have been much more rapid. But discoveries and inventions are 

 apt to appear obvious when regarded from the standpoint of 

 accomplished fact ; and I draw attention to the matter only to 

 point the moral that we do well to push the attack persistently 

 when we can be sure beforehand that the obstacles to be overcome 

 are only difficulties of contrivance, and that we are not vainly 

 fighting unawares against a law of Nature. 



The present development, of electricity on a large scale depends, 

 however, almost as much upon the incandescent lamp as upon the 

 dynamo. The success of these lamps demands a very perfect 

 vacuum — not more than about one millionth of the normal 



