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unfortunate. It is certainly not true but it was used through the eighteenth century and the first half of the nineteenth century to support the argument that acoustic fog warning signals would be useless. The degree of certainty in Derham’s words is surprising particularly in contrast to the uncertain- ty throughout this section with regard to other effects. When Derham includes detailed observations, he does not shrink from pointing out inconsistencies. In this section alone, he uses the phrases, “...but this is not uniformly and always the case...”, “...but the contrary often happens...” In contrast, he gives no observational evidence regarding propagation in thick fogs, just the unequivocal statement. One wonders whether he is simply repeating “what everyone knows.”] For sounds then seem to be for the most part very weak and blunted—a fact which very certainly proceeds from the inter- posed vapors and thick particles which compose fog. I have likewise observed the same concerning snowy weather. [Some authors assert that Derham believes that falling snow damps sound but the next paragraph makes it clear that Derham is talking about the effects of snow on the ground. Freshly fallen snow is a poor reflector of sound.]
For when fresh snow has fallen on the ground sounds straightway grow dull; but when its surface has been covered with ice, the sounds suddenly become more acute, and I then have heard bells ringing and cannon booming just the same as if there was no snow on the ground. My friend Towneley was telling me not very long ago that he had observed (the like of which I have myself experienced) that when he was riding on horseback ringing not far from him was barely able to reach his ears whenever a house covered with snow lay between him and the sound, so that he on entering the little town, was very much surprised that the bells should so sud- denly be stilled while he was passing along the first houses that intervened, and that they should suddenly sound again when he was passing along the next vacant space. Indeed during the whole of his course in this town he observed that the sound of bells reached his ears or not according as build- ing covered with snow were intervening or not.
But concerning these things more than enough has been said. We proceed to other matters of greater moment.
11. Concerning the force of winds or their influence on the velocity of sound.
The illustrious Accademi del Cimento at Florence found from experiments that the velocity of sound was neither retard- ed by adverse winds nor accelerated by favorable ones, but that, however the winds might blow, sounds always traversed the same space in the same time. Gassendi was of this opinion, and almost all the rest who have philosophised before or since.
Since, however, the contrary of this is plain from mere experience, these authorities must be corrected of error, into which they seem to have fallen for this reason, that their experiments were tried within a too short space. For it is very probable that these philosophers made their observations at a distance of only one or, at the most, of two or three miles. Hence I do not wonder that their observations are faulty; but if they had tested the matter, as I have often done, at ten or twelve thousand paces, using accurate instruments, they
would have easily recognized their error.
This common error, I myself, relying on the authority of
these men, admitted for a long time, until at length, by more than three years of observation of cannons on the Blackheath grounds, I luckily detected it. When, however, at first I per- ceived the sounds to come to my ears sometimes quicker, sometimes slower, the suspicions entered my mind that I had committed some error, either because I had less accurately counted the vibrations of the clock, or had badly observed the flash of the cannon, or from want of attention, had fallen into some other such like error. But after the cannons were continuously fired, at my request, every half hour, from six o’clock in the evening till midnight, and after I constantly perceived that the sound reached me, without any perceiv- able variation, in the space of a hundred and twenty or of a hundred and twenty-two half seconds, however much wind may have been directly adverse; while at other times, when the wind was blowing favorably, either directly or crosswise or obliquely, I found that the sound of the same cannon reached me in the space of 111, 112, 113, 114, 115, 116 or, at the most, of 117 half seconds, then at last I became thor- oughly persuaded that there was a certain real difference that produced this variety in the observations.
Nor is it only true that favorable or adverse winds accel- erate or retard the velocity of sounds but it is also true that, in accordance with the variety of the degrees with which they blow more strongly or more gently, so much the more or less do they promote or impede this velocity. For greater certain-
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