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usual passage. So, in fine, on Sept. 29, 1705, when a stronger and less favorable wind was blowing, the sound completed its progress within 112 half seconds. From which examples, and from others in the table, it plainly appears that stronger winds assist the propagation of sound, but that lighter winds are less effective in promoting its propagation.
The same likewise is plain respecting those winds or cur- rents of air which directly favor or obstruct the progress of sound—that they make its velocity quicker or slower—and where intermediate currents (streams) of air are blown, that they produce in like manner an intermediate progress of sound, as measured by the vibrations of the pendulum.
The greatest difference which I have yet observed in the passage of sound through a space of about 13 miles amounts to about nine or ten half seconds, that is, when strong winds are aiding and only gentle ones are obstructing the sound.
But when only gentle winds, or almost none at all, are either obstructing or aiding the sound, then the difference does not exceed two or three half seconds.
After, in this way, I had perceived what influence the winds have, both for accelerating and retarding the course of sounds, curiosity led me to inquire into the velocity of the winds themselves. And though the inquiry may be foreign to my subject, it will not be wholly ungrateful, as I hope, to curi- ous minds, if I publish in this connection certain observa- tions on this point.
12. Concerning the velocity of winds.
In order to ascertain how large a space winds may tra- verse in any given time, I have used, in prosecuting my exper- iments, certain bodies of the somewhat lighter sort, such as thistle down, light feathers, etc., which seemed better to serve my purpose than the instrument which is described for us in the Philosophical Transactions, No. 24; or even that other more available one, recalling the figure of a mill with wings attached, invented, unless I mistake, by our most acute friend, the late Dr. Hook. [Derham often references the Philosophical Transactions by number rather than by vol- ume. The reference here is to Moray and Hook, “Directions for observations and experiments to be made by Masters of ships, Pilots, and other fit persons in their sea-voyages,” Phil. Trans. Royal Soc. London, 2, 433-448, 1667.]
From very many experiments which I have made, with the aid of the lighter sort of bodies, when the winds were blowing with different degrees of force, I have found that the most violent wind traverses scarcely 60 miles an hour. For example, on the 11th of August, 1705, the violence of the wind excited such a tempest that it almost overturned the windmill itself near the spot where I made my observations. [The different degrees of the force of the winds, as has just been seen, I have for the most part noted by these numbers: 0, 1, 2, 3, 4, 5, 6, up to 10, 15, or still higher degrees.] Now I have estimated that the force of the above indicated wind answers to about 12 or 14 on this scale. And from very many reiterated experiments I have concluded that that violent wind traversed about 33 feet in a half second, or 45 miles in an hour; hence I gather that the fleetest and most tempestu- ous winds (that violent wind which raged in the month of
November, 1703, not being excepted) do not traverse more than 50 or 60 miles an hour.
After we have measured the velocity of the rapid winds, it is not difficult to conjecture what may be the velocity of less rapid ones; for I have also marked the course of these, and from various experiments I have convinced myself that some of them accomplished 15, some 13, others many less miles per hour; while some are propagated with such a slow motion that they move scarcely a single mile an hour. Moreover, other winds are so sluggish that one may easily outstrip them while making a journey on horseback or on foot. This fact is apparent to our senses, for when we arrest our steps we per- ceive a soft breeze gently fanning us, but if we advance with it we feel none at all; while if we quicken our pace instead of a breeze accompanying us and blowing in the same direction with our movement, we plainly feel the air resisting us, and blowing full in our faces. Likewise when the atmosphere is entirely quiescent and stagnant, if we chance to be walking or riding on horseback, we then perceive a gentle breeze press- ing against us, with such degrees of force, in fact, as corre- spond to the rates of our own motion. And a breeze of wind or current of air is borne with the same rate of motion or velocity when it presses against us with an equal impetus as we stand still, or linger in our track.
From these observations about the velocity of winds very many things, not without utility, might be noted, but espe- cially might we assign in view of them, one reason why the mercury rises and falls for such a long time before clear weather or rain sets in.
But I will omit these considerations as being foreign to my purpose, and this only will I observe as to sounds, to wit, that while their motion is accelerated by wind it is plain that those parts of the atmosphere by which sounds are impressed or propagated are not the same as those from which winds are blown, but certain other more ethereal and volatile parts, as one may suppose. For the fleetest winds do not pass through more than 60 miles in an hour, but sounds travel more than seven hundred thousand paces, or 778 miles in the same time.
But if it be objected that winds do accelerate or retard sounds it is to be answered that this does not only proceed from the current or tendency of the windy particles alone, but rather from the conjoint and cooperating motion of all the particles of the atmosphere, both the thicker and the ethereal. If the direction of this course of motion favors the waves of sound it is altogether in accordance with probabili- ty that the impulse of sounds should be accelerated by this cause, but if the direction is adverse that, the impulse should be retarded.
13. Concerning the velocity of sounds.
After having in this way set forth the operation and effects of wind on the progress of sound and having spoken thus generally respecting the velocity of sounds, it remains at last that I should report the more special observations which I have made on this point. From what has been said, there- fore, and from many other things which we have noted before I conclude most decisively that sounds are propagated with
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