Page 33 - January 2009

echo came from afar] is not contrary to the laws of the echo. In the second place it is to be remarked that this double sound came directly from Blackheath, for, of the two sounds, the former did not come from that quarter, and then a second sound, (after the manner of an echo), came from somewhere else, i.e., either beyond me, to the right of me, to the left of me or from any different direction – I have often observed the same thing when big guns were fired from ships in the river Thames, (especially if the air was clear and still), at the evening and in the morning where the watch-guns are discharged. After the crash of the gun had struck my ear I have heard it running far away along the river, and booming through many miles from the bank, from the mountains, and from the rocks thickly studded along the Kent county coast. “These all”, says my friend, “come from the reverberation of the houses, etc., near you.” But to say nothing respecting the weakness of a sound after it has traversed very many miles, and respecting its inca- pacity if it has come so far, to be rebounded by sound reflecting objects near the observer, rather than by sound reflecting objects situated near the sounding body – to say nothing about these things, I will give one or two examples from which it will plainly appear that an echo made by sound-reflecting objects near the sounding body can be heard through many miles as well as the primary sound— sometimes even clearer than the latter.
I have often observed that the heavy guns fired from ships in the Thames at evening about Deptford and Cuckold’s Point, send forth for the most part a crash that is duplicated, triplicated, quadruplicated, or even still more multiplied, and that the later crashes are more sonorous. And when I have gone here and there a hundred and twenty five paces, or even a quarter of a mile, or half a mile in a transverse direction, the sound was still the same. I remember that on the 8th of last March many cannon were fired, somewhere between the aforesaid Deptford and Cuckold’s Point, from a ship in the Thames, which I saw from my church. The crash of these was repeated five or six times, in this manner:
Between the flash and the sound I counted 122 half seconds – the wind blowing crosswise. At that time, therefore, the guns were distant from me more than 13 miles. The first two reports were much weaker than the third reports, but the last reports were much the most sonorous of all. And when I had gone across a quarter of a mile towards the right, there was still the same multiplied sound, and when towards the left, it was still the same. And, moreover, in some of my halting places, besides the multiple sound, I clearly heard a weak echo reverberated from my church or the adjacent houses. Indeed, I then made the same observations, as often as the guns were fired.
Another observation of this kind was made on a certain Sunday, about two or three years ago, from the sound of a heavy cannon fired somewhere on the river Thames, this side or beyond the town of Gravesend. The crash of this gun was multiplied at least 8, 9, or 10 times, according to this score:
Very many persons who at that time were on their way to Divine Worship supposed that this multiplied sound was the crash of many guns from a ship engaged in battle but, as I think, it was nothing else than a many-voiced echo from the sound of a single gun as it was fired, or from the sound of another and another reverberated by the many adjacent ships or by the shore. What makes for my opinion is the fact that I was not the only one who heard it (I was walking at the time in my gar- den)—but also many other persons who were far distant. Mr. Barret likewise an ingenious and learned member of our Royal Society, heard the same repeated sound at his house at a dis- tance of about four miles from Upminster, where I heard it.
From all these considerations it clearly appears that the opinion of my aforesaid friend (so worthy to be respected on many accounts) is false.
3. Concerning Echo or the Reverberation of Sounds in the Air.
To these remarks on echoes I hope it will not be unac- ceptable to add an illustration from the reverberation of sound by aerial particles, which will confirm what has been said.
When I have heard the crashes of heavy artillery, espe- cially in a still and clear atmosphere, I have often observed that a murmur high in the air precedes the report. [Similar observations were made in the second half of the nineteenth century by Joseph Henry. Henry provided much observa- tional evidence to support the idea that winds bent the path of sound (refraction) through that atmosphere—an idea that was foreign to any of the philosophers of Derham’s era. But Henry also observed what appeared to be echoes from the air above and these were a source of great curiosity to Henry and others to follow.] And in thin fog I have often heard the sound of cannon murmuring in the air high above my head through many miles, so that this murmur has lasted 15 sec- onds. This continuous murmur, in my opinion, comes from particles of vapor suspended in the atmosphere which resist the course of the sound waves and reverberate them back to the ears of the observer, after the manner of undefined echoes, which we call a murmur in the air. [This explanation presages John Tyndall’s explanations for many of the curious phenomena associated with sound traveling through the atmosphere. However, in many cases, Tyndall, a critic of the idea that wind and temperature gradients could bend paths of sound, was subsequently shown to be wrong.]
Where these facts are fully weighed it will be manifest that echoes made at a distance are capable of being heard, and that that aforesaid reduplication of the crash of artillery on Blackheath came without doubt from Blackheath itself, as I have just asserted.
4. Concerning the Sounds of gunshots fired in every angu- lar direction, etc.
To return from this digression concerning the reverber- ation of sounds, I will proceed to my observations concern- ing their velocity—observations which I have derived from
32 Acoustics Today, January 2009