Page 39 - January 2009

dantly clear that sounds can be heard much further in Italy than my before named ingenious friend informed me. For the Excellent Envoy himself has heard the report of heavy guns at the distance of sixty miles. The guns likewise which at his request were fired at Florence and were heard 55 miles. The cannon fired at Leghorn were audible at a distance of 60 miles. Those which were discharged at Messina, as appears from the geographical tables surprised the ears of men who were distant nearly 100 Italian miles. The report of those which were fired in the siege of Genoa traversed (as appears from the maps) more than 90 Italian miles [one Italian mile = 1.0277 English miles].
When all these facts are recalled to the mind and seri- ously weighed, I can scarcely avoid the belief that sounds are propagated no less widely in all quarters of the south than in the northern parts of the world. Although the examples of a greater progress of sound are not wanting in certain northern quarters of the globe, a Danish nobleman, a servant of our illustrious Prince of Denmark, has told me, in a conversation, that while he was living in Denmark he had clearly heard the report of cannon fired at Carlscroon, a distance, unless my memory deceives me, of 80 English miles. Likewise that very skilful man Dr. Hearn, the physician of the most illustrious King of Sweden has communicated a special memoir to our Royal Society respecting the guns fired at Stockholm when the obsequies [a ceremony to honor the recently departed] of one of the Royal Princes were celebrated in the year 1685, the report of which traversed an interval of 30 Swedish miles, which are equal to about 180 English miles. At that naval bat- tle likewise which was fought between England and Holland in 1672, the sound of the cannon struck the astonished ears of men through an intervening space of more than 200 miles, reaching as it did, across our island even as far as Salisbury and Wales.
What therefore both the Towneley brothers observed is wholly special and peculiar to the aforesaid Castle of St. Angelo or at least to Rome. For it is not permitted to suspect either the perspicacity of their intellects or their conscien- tious carefulness.
The dimunition of sound which they observed (unless I surmise amiss) must be ascribed either to the situation of this aforesaid castle, or to the intervening houses (rising every- where and on all sides in that very crowded city) or to the din of the city resounding from all quarters, or to adverse winds, or, in fine, some other like cause, which I leave to be attained by happier conjecture on the part of those whose lot it is to live there, or perhaps these men made their aforesaid obser- vations in that state of the atmosphere in which sounds, although they have the most favorable winds, are neverthe- less much more languid than at other times when the winds are entirely adverse. And at one time I had persuaded myself that an atmospheric condition of this kind always obtains at Rome, and not in other parts of Italy, until I fell upon the contrary opinion of Kircher, who says: “Here at Rome, won- derful to relate, when the north wind blows, echo or sound acquires the greatest vigor; when the south wind blows it is weak; when the southeast and east winds blow, it is of a medi- um character.”
But this condition of the air, since it affects sounds so greatly, will not be foreign from our purpose to consider in detail. It is my purpose, therefore, in the next place to treat it more fully, and to set forth the observations which I have made in the premises.
10. Concerning the various weakening and intensity (or audibility) of sound according to the different state of the atmosphere.
I have often observed in summer time, when the air has grown hot, that sounds appeared more languid than usual and were exceedingly weak in their impression on the ear; while in weather of another sort, especially in winter, if it happens to be freezing cold, the same sounds were much more piercing and shrill, and struck the ear more forcibly. Also, when the north or south east wind was blowing, how- ever adversely, I have observed the sounds to be clearer and shriller than if the wind was blowing from contrary quarters, as Kircher also observed at Rome. But this is not uniformly and always the case.
Nor could I form any more certain conclusions from the inspection of a rising or falling barometer as I had too confi- dently expected to do. For I discovered that sounds were sometimes very clear and shrill, sometimes very faint and languid when the mercury was rising to the top; and on the contrary, sometimes very strong, sometimes very weak, when the mercury was sinking to the bottom.
A like uncertainty obtains with regard to clear and foggy air. In rainy and damp weather I have often observed that sounds are blunted and that after torrential rains they acquire the greatest strength, as Kircher observed at Rome. But the contrary also often happens. For instance, on May 31, 1705, the air on this occasion was more clear and free of vapor than I remember ever before to have seen it. For such was the puri- ty and liquid serenity of the atmosphere that I could clearly and readily perceive exceedingly remote objects. But never- theless, I was not able to hear the cannon that were fired at that time on the grounds of Blackheath (if I except a single one whose thud reduced to a faint sound, I may have caught in my ears), although I could clearly perceive the flash of them all in the distance. And at the same time the current of the clouds and of the wind was setting in the same direction with the sound. Moreover, the breeze which was then blow- ing was a very gently one, such as could scarcely ruffle the hair; and, in fine, all things necessary to promote the force and velocity of sound seemed to concur. But on the other hand, when the state of the air and weather has been wholly changed—when everything seemed turbid and the atmos- phere full of vapor—I have often heard strong sounds, and not less often have I heard them blunt and weak.
The causes of these variations I leave to be inquired by others, since I confess that it equally exceeds the grasp of my mind to discover them and to assign what may be the proper medium or vehicle of sound – whether the ethereal and more subtle part of the atmosphere, or the vaporous and denser part of the atmosphere, or both combined.
But as regards thick fogs, it is certain that they are dampers of sound in the highest degree. [This statement is
38 Acoustics Today, January 2009