222 
PROFESSOR TYNDALL ON THE ATMOSPHERE 
morning we had a light air from the N.E., force 1. Haze over the sea; not deep, for 
the sun shone through it : water very smooth. 
About a quarter of a mile from the pier end, and at 2‘6 miles from the station, we 
waited, and heard distinctly the tuning of the horns. Never previously had they been 
heard so loud in this position — a result plainly due to the shifting of the wind. 
At 10 a.m. the gun was good; the horn was also good, voluminous and musical; the 
syren was very loud, hard and penetrating. This was the result of repeated obser- 
vations. 
Steamed towards Dover Castle into the sound-shadow, on entering which all the 
sounds were greatly enfeebled ; but the syren, though vastly fallen, remained distinctly 
superior to the four horns. 
We manned the gig and rowed towards the shore. Halted near the end of East 
Cliff Terrace, where the horns and the syren were audible, but barely so. The sounds 
at times rose and sank in power, so that after one had been set down as barely audible, 
a succeeding blast rendered a qualification of the statement necessary. The sound 
appeared to be a little feebler at some distance from the beach than close to the beach 
itself. 
It was rendered certain by these observations that the four horns, two of which were 
on the summit and two at the base of the cliff, had in the sound-shadow no advantage 
over the syren. 
A gun was fired at 10.30 ; but we were not attending to it, and no one in the gig heard 
it. In the ‘ Galatea,’ a quarter of a mile nearer the edge of the acoustic shadow, the 
report was heard as a feeble thud. Admiral Collinson, however, thought it had an 
advantage over the syren. 
Steamed round the Foreland along an arc of 2 miles radius. I happening to be in 
the deck-cabin, heard there a sudden and powerful augmentation of the sound of both 
syren and horns ; at the same moment Mr. Edwards came to me to announce that the 
4 Galatea’ had just cleared the sound-shadow, the instruments being in sight. The 
shadow was therefore sharply defined. This sharpness was repeatedly observed on both 
sides of the Foreland. 
Steamed towards the axis ; but before we reached it the syren-sounds rose to an extra- 
ordinary degree of power, the horns yielded a full and mellow sound, far, however, below 
the syren in intensity ; both were well heard through the noise of the paddles. 
At 11 a.m. near the axis, distance 2 miles, the gun was fired, and yielded a very loud 
report. Steamed out along the axis; for a time both horns and syren were heard 
through the paddle-noises, but at 3 miles distance the horn-sounds vanished from the 
hearing of some, while a feeble murmur from them continued to be heard by others. 
To all, however, the syren was by far the clearest sound. 
At 11.30, our distance being 5^ miles, the gun was fired, but no sound was heard. 
At this hour the gun was clearly mastered by the syren. At 7 miles distance, the 
paddles still going, the syren yielded a distinctly serviceable sound : to a_sailing-vessel 
