202 
PROFESSOR TYNDALL ON THE ATMOSPHERE 
this association suggesting that the duration of the echo is a measure of the atmo- 
spheric depths from which it comes. On no day, it is to be remembered, was the 
atmosphere free from invisible acoustic clouds ; and on this day, and when their presence 
did not prevent the direct sound from reaching to a distance of 15 or 16 nautical miles, 
they were able to send us echoes of 15 seconds duration. 
It remains to be added that on many occasions when the ‘ Galatea ’ was 2 miles and 
on some when she was 3 miles from the shore, with the Foreland bearing north, distinct 
and long-continued syren-echoes were sent back to us from the transparent southern air. 
On various other occasions, when the atmosphere was exceptionally pure, we have 
fired the e Galatea’s ’ guns with a 1-lb. charge ; echoes were always returned from that 
portion of the atmosphere towards which the gun was pointed. 
Several times also during the prevalence of wind the ship has been turned across the 
wind and its two guns fired, the one to windward, the other to leeward. On all occa- 
sions the echoes of the leeward-pointed gun were distinctly more powerful and long- 
continued than those of the gun fired to windward. 
To sum up this question of aerial echoes. The syren sounded three blasts a minute, 
each of 5 seconds duration. From the number of days and the number of hours per day 
during which the instrument was in action we can infer the number of blasts. They 
reached nearly twenty thousand. The blasts of the horns exceeded this number, while 
hundreds of shots were fired from the guns. Whatever might be the state of the weather, 
cloudy or serene, stormy or calm, the aerial echoes, though varying in strength and dura- 
tion from day to day, were never absent ; and on many days “ under a perfectly clear sky ” 
they reached, in the case of the syren, an astonishing intensity. 
§ 9. Experimental Demonstration of the stoppage of Sound by Aerial Deflection. 
The stoppage of sound by aerial reflection has never been experimentally demon- 
strated ; it has been hitherto a matter of inference, and I wished to reduce it to experi- 
mental demonstration. A few preliminary experiments satisfied me that a carefully 
constructed apparatus would be necessary for the purpose. 1 therefore requested my 
assistant, Mr. Cottrell, who is eminently skilful in devising apparatus the object of 
which has been made clear to him, to make an arrangement by which alternate layers 
of carbonic acid and coal-gas, the one falling by its weight, the other rising by its 
lightness, should be obtained. After making in the first instance a rough model 
himself, he invoked the aid of an intelligent carpenter, and produced the instrument 
represented in section and plan in figs. 1, 2, & 3, Plate XVIII. 
XY (fig. 1) is the sectional elevation of a wooden tunnel with a glass front. B is a 
small bell enclosed in a carefully padded box with one opening, worked sometimes by 
electricity and sometimes by a small magneto-electric engine. The letters a , b, c, d, e , 
&c. indicate the front view of a series of pewter tubes about f of an inch wide, bent over 
at the top, and inserted into a box behind X Y. The box is seen in plan at rs, fig. 2. 
Into it is inserted the tube mn , with an orifice and trunk-tube at o, this trunk-tube 
