132 POPULAR LECTURES AND ADDRESSES. 



large disc, driven at a uniform l rate by wheelwork, 

 and the blast supplied from a steam boiler, or from 

 a large vessel of compressed air, sustained by 

 powerful condensing pumps. I am informed that 

 recently an improvement has been made in this 

 country by substituting a rotating cylinder for the 

 rotating disc. 



69. Professor's Henry's experiment made for 

 the United States Lighthouse Board, of which 

 he is chairman, showed that the siren was much 

 superior to the powerful fog-horns and steam 



1 It seems that improvement in respect to this quality is needed 

 in the instruments hitherto made. In some of the reports of the 

 experiments, I see it stated that the pitch of the sound gradually 

 fell when the siren was kept sounding continuously for some time ; 

 because the steam pressure in the boiler diminished, and so the 

 rotating disc ran slower. The rotating disc ought to be kept running 

 with almost chronometric uniformity. There is not the slightest 

 difficulty in doing this by having it driven either by a constant 

 weight, or by aid of a proper slip gear adapted to drive with con- 

 stant force. With this and a proper centrifugal governor, there is 

 no difficulty whatever in securing so nearly perfect uniformity that 

 the rate shall never alter by as much as I per cent. This would 

 produce not more than 1/4 of a semitone of difference in the pitch of 

 the note. The power required to turn the disc is so very moderate 

 that there is absolutely no difficulty in realising the improvement I 

 have now suggested. Possibly the best plan will be to drive it by 

 manual power. One man amply suffices for the purpose. 



