594 PHILOSOPHICAL TRANSACTIONS. [anNO I7O8. 



mortars and shells. 4. To the geographer, for measuring with the greater ease 

 and certainty the distances of places ; for any one furnished with a large quan- 

 tity of gunpowder, may by this means, in an hour or two, make a very accurate 

 map of almost a whole country : the report of guns will show, as has been said, 

 the distances of the places ; and any mathematical instrument that measures 

 angles, as the plane table, &c. will give their situation, so that they may after- 

 wards be easily delineated ; and in like manner, by this means may any one 

 readily dii<cover the justness and accuracy of maps, and correct their errors : in 

 short, this method of observation may be of singular service in measuring in- 

 accessible distances, especially very broad rivers, &c. that cannot be otherwise 

 measured ; as also for finding the breadth of bays and straits. 

 -" 5. To the echometrician ; though several learned men, both ancient and 

 modern, have carefully examined into that ludicrous and agreeable phenomenon 

 of sound, called echo, yet they are not well agreed in a great many points re- 

 lating to it ; especially as to the space necessary for the repetition of J, 2, 3, or 

 more syllables, or, which is the same thing, the space an echo moves through 

 in any given time ; Mersenne allows .... paces, Blancanus 24 paces, with 

 whom agrees Dr. Plot; but Kircher asserts, that nothing certain can at all be 

 determined therein ; because the variation of the winds, the intenseness and 

 remissness of sound, and a great many other circumstances may cause a great 

 difference. It is an easy matter to assign the reason of the variety of these ob- 

 servations : as, the slowness and various disposition of our senses, the different 

 audibility of sound, the grave or acute sound of the syllables themselves, or 

 their length or shortness, or some other cause, that protracts the time of their 

 pronunciation ; for I am persuaded, that though any reflecting object w«re 

 capable of returning all the syllables of the following verse; vocali nymphae, 

 qua nee reticere loquenti, yet it could not reflect all the syllables of this other, 

 because its pronunciation is a little longer ; corpus adhuc echo, non vox ©rat, 

 et tamen usum ; and much less repeat all the rough and long syllables of the 

 following verse, though fewer in number ; viz. 



^n, tridens, rostris, sphinx, praester, torrida, seps, strix. 



But from the abovementioned observations on the motion of sound, we may 

 conclude, that as sounds, so do echos in like manner move through certain 

 and determinate spaces in given times, as I have often found by experience; 

 viz. that the echo returns in double the time wherein the primary voice arrives 

 at the reflecting object : for instance should the phonocamptic, or reflecting 

 ^object,' be at the distance of a furlong, the return of the echo will he made in 

 the same time, that the primary sound wpuld move through two furlongs, if it 



