220 
PROFESSOR TYNDALL ON THE ATMOSPHERE 
of an act of condensation, which renders homogeneous the acoustically flocculent or 
turbid air. The fog-particles appear to have no more influence upon the waves of 
sound than the suspended particles stirred up over the banks of Newfoundland have 
upon the waves of the Atlantic. 
§ 15. Atmospheric Selection. 
It has been stated in § 3 that the atmosphere on different days shows preferences to 
different sounds. This point is slightly touched upon in the record of the Serpentine 
observations ; but it is worthy of further illustration. 
After the violent shower which passed over us on October 18th, the sounds of all the 
instruments, as already stated, rose in power ; but it was noticed that the horn-sound, 
which was of lower pitch than that of the syren, improved most, at times not only 
equalling, but surpassing the sound of its rival. From this it might be inferred that 
the atmospheric change produced by the rain favoured more especially the transmission 
of the longer sonorous waves. 
But our programme enabled us to go further than mere inference. It had been 
arranged that up to 3.30 p.m. the syren should perform 2400 revolutions a minute, 
generating 480 waves a second. As long as this rate continued, the horn, after the 
shower, had the advantage. The rate of rotation was then changed to 2000 a minute, 
or 400 waves a second, when the syren-sound immediately surpassed that of the horn. 
A clear connexion was thus established between aerial reflection and wave-length. 
The 10-inch Canadian whistle being capable of adjustment so as to produce sounds 
of different pitch, on the 10th of October I ran through a series of its sounds. The 
shrillest appeared to possess great intensity and penetrative power. The belief that a 
note of this character (which affects so powerfully, and even painfully, an observer close 
at hand) has also the greatest range is a common one. Mr. A. Gordon, in his exami- 
nation before the Committee on Lighthouses in 1845, expressed himself thus : — “ When 
you get a shrill sound, high in the scale, that sound is carried much further than a 
lower note in the scale.” I have heard the same opinion expressed by other scientific 
men. 
On the 1 4th of October the point was submitted to an experimental test. It had 
been arranged that up to 11.30 a.m. the Canadian whistle, which had been heard with 
such piercing intensity on the 10th, should sound its shrill note. At the hour just 
mentioned we were beside the Varne buoy, 7f- miles from the Foreland. The syren, as 
we approached the buoy, was heard through the paddle-noises; the horns were also 
heard, but more feebly than the syren. We paused at the buoy and listened for the 
11.30 gun. Its boom was heard by all. Neither before nor during the pause was the 
shrill-sounding Canadian whistle once heard. It was now adjusted to produce its ordinary 
low-pitched note, which was immediately heard. Still further out the low boom of the 
cannon continued audible after all the other sounds had ceased. 
But it was during the early part of this day only that this preference for the longer 
