1874] WRITINGS OF JOSEPH HENRY. 395 



Comparison of the Daboll trumpet and the siren. — The pres- 

 sure of the hot air in the reservoir of the hot-air engine of 

 the trumpet was about 20 pounds, and that of the steam in 

 the boiler of the siren about 75 pounds. These pressures 

 are however not considered of importance in these experi- 

 ments, since the object was not so much to determine the 

 relative amount of motive power employed, as the amount of 

 penetrating energy produced by these two instruments, each 

 being one of the first of its class. 



1. At distance 50, the trumpet produced a decided motion 

 of the sand, while the siren gave a similar result at distance 

 58. The two observations being made within ten minutes 

 of each other, it may be assumed that the condition of the 

 wind was the same in the two cases, and hence the numbers 

 above given may be taken as the relative penetrating powers 

 of the two instruments. 



2. Another series of experiments was instituted to deter- 

 mine whether a high or a low note gave the greater pene- 

 tration. For this purpose the siren was sounded with 

 different velocities of rotation of the perforated disk, the 

 pressure of steam remaining at 90 pounds per square inch. 

 The effect upon the artificial ear in causing greater or less 

 agitation of sand was taken as the indication of the pene- 

 trating power of the different tones. The number of revolu- 

 tions of the disk in a given time was determined by a count- 

 ing apparatus, consisting of a train of wheels and a series of 

 dials showing tens, hundreds, and thousands of revolutions; 

 this was temporarily attached to the projecting end of the 

 spindle of the revolving disk by pushing the projecting axis 

 of the instrument into a hole in the end of the spindle. 



From the whole of this series of experiments it appeared 

 that a revolution which gave 400 impulses in a second was 

 the best with the siren when furnished with a trumpet. On 

 reflection however it was concluded that this result might 

 not be entirely due to the pitch, but in part to the perfect 

 unison of that number of impulses of the siren with the 

 natural tone of the trumpet. To obviate this complication 

 a series of experiments was next day made on the penetra- 



