Page 43 - January 2009

the following degree of velocity, to wit, that they traverse the space of a mile or 5280 English feet in 9 1⁄4 half seconds. Or which amounts to the same thing, 571 ft in 1⁄2 second and 1142 ft in a whole second.
This, however, is the defined space traversed by sounds, if a current of the atmosphere blows across their path, and is their mean progress or velocity.
But if the wind increases the rapidity of the sound, it is possible that it may traverse more than 600 ft in the space of a 1⁄2 second; or on the other hand if the wind retard its motion it may not proceed more than 560 ft in the same interval of time.
So, at length, I have brought to an end this memoir of mine, in which I have summarily embraced the principal observations I have made about the progress of sound, and certain other things pertaining thereto.
Practiced and ingenious men will not have much diffi- culty in applying this exposition to very many uses which are not to be despised. But especially would the aforesaid obser- vations and experiments seem to pertain not a little to
1. The Philosopher, who, even because of them, will be in some respects better equipped for the investigation of the secret nature of sounds and for explaining their very numer- ous abstruse phenomena;
2. To the Sailor, who hence may learn how far off are the ships which he sees floating in the distance, or lying at anchor; how remote likewise may be the desired land or beach which he sees in the distance—facts which, from shots designedly fired on a given signal, may be easily and certain- ly known;
3. To the Soldier for the purpose of finding how far an enemy has placed his camp; at what distance an arsenal, a fort, or a besieged city is situated, for the purpose of planting against them siege cannon, and aiming mortars and bomb shells;
4. To the Geographer, for more readily and certainly measuring the distances of places, because any body who is furnished with a small quantity of powder can, in this way, within an hour or two exhibit almost the whole of any region with a table most accurately outlined seeing that gunshots, as I have said, serve to mark distances by their firing, and any mathematical instrument by which angles are measured, either that common instrument which surveyors use, called the Plain Table or a single rule, furnished with graduated scales, will indicate the situations of the various places, which afterwards can be easily delineated. [The Plane Table or Plain Table was normally used in surveying as follows: two or more level tables would be set up at surveyed locations. A sighting device—an alidade—was used on each table to draw direc- tion lines to objects sighted from both locations. From the sighting lines from both tables, and the locations of the tables, a map could be constructed locating any object sight- ed.] In this way, too one can readily inquire into the correct- ness and truth of maps, and if they have any errors he can correct them.
In fine, this method of observing sound, would be of great use in measuring distances of inaccessible places, espe- cially of very wide rivers and places of that kind not other-
wise easy to be measured. For a specimen of this work, I resolved with the aid of friends to compare the distances of certain among the more celebrated bays and straits, especial- ly of the Strait of Gades, between Tangier and Gibraltar, and the British Channel between Dover in England and Calais in France, where the breadth of the channel according to the measurement of ingenious Frenchmen, is 22.07 English miles. But the lamentable season of war through which we are now passing has interposed an obstacle to these under- takings and to others having for their object the promotion of learning;
5. For the measurement of echoes. Although many learned men have anxiously inquired both anciently and in subsequent times concerning this amusing and pleasant phe- nomenon of sound, still there is not a good degree of harmo- ny among them respecting many things which relate to it, especially respecting the extent of space necessary for the repetition of one, two, three or more, syllables, or, what amounts to the same thing, respecting the space traversed by an echo in a certain interval of time. Mersenne allows... [ellipsis in Latin manuscript] yards for the repetition of a monosyllabic sound. Blancanus allows 24 yards, to which our very celebrated countryman Dr. Plot gives his assent, but Athanasius Kircher asserts that nothing at all can be defined with certainty respecting it, because, the variation of the winds, the intensification and the relaxation in the force of sound and many other things produce an immense variation.
It is not difficult, however, to offer an explanation of this disagreement among these distinguished men, for it can arise from many causes—certainly from the slowness and from the different disposition of our senses, or from the various audi- bility of sounds; from the grave or acute sound of the sylla- bles themselves, or from their protracted or prolonged pro- nunciation, or from any other cause which may protract the interval of time. I can have no doubt for instance, that if any sound reflecting object, should be able to reverberate all the syllables of this verse
vocalis nymphe quae nec reticere loquenti
[Welling’s note pertaining to this line of Latin and the next: Quoted from Ovid’s Met., iii, lines 357 and 359.]
it would not be able to reverberate all the syllables of the following verse, since its pronunciation is much more pro- longed:
corpus adhuc echo, non vox erat, et tamen resume
And still less would it be able to repeat all the harsh and prolonged syllables of the following verse:
Arx tridues rostris sphinx, praester, torrida, seps, strix.
But from the foregoing observations concerning the velocity of sound it may be concluded that echoes, like sounds, traverse certain and determinate spaces in a certain definite time. What I have myself frequently learned from
42 Acoustics Today, January 2009