Olfefvations on Sound and jicotifiic Itijlrmnents. 4T7' 



i. As found pafTes through the air with a velocity of 1142 feet in a fecond, and the 

 fwifteft wind does not pafs with a velocity of 90 feet in the fame time ; it might naturally 

 be inferred that the velocity of found with or againfl the wind ought to be fomewhat af- 

 fected, but its intenfity very little. The contrary, however, is the cafe; from which, and 

 other circumftances, fome philofophers have been difpofed to conclude that the medium 

 by which found is tranfmittcd is not air, but another fluid of greater fubtilty. 



3. Sounds feem more intenfe, and are heard to a greater diftance, by night than by day. 

 In a Hill night, the voices of the workmen at the diftillery at Batterfea may be heard at 

 Weftminfter Bridge, over an interval of about three miles ; the watch-word at Portfmouth 

 may be heard at Ride in the Ifle of Wight, the diftance of which is between four and five 

 miles ; and numerous inilances are related of the propagation of weak founds to much 

 greater diftances, without mentioning thofe of which the intenfity is greater. It is a prac- 

 tical queftion of fome importance to afcertain whether this difference may arife from the 

 different ftate of the air, the greater acutenefs of the organ, or the abfence of the ordinary 

 noifes produced in the day. By attentive liftening to the vibrations of a clock in the night, 

 ■and remarking the difference between the time when no other noife was heard, and when 

 a coach palTed along, it has appeared clear to me that this difference arifes from the greater 

 or lefs ftillnefs only, and that no voluntary effort or attention can render the near found 

 much more audible, while another noife acls upon the organ. In this fituation the ear is 

 nearly in the (late of the eye, which cannot perceive the ftars in the day time, nor an ob- 

 je£l behind a candle. 



4. It would be. impoffible to ufe optical inftruments in the day time, if the light of other 

 objefts befides thofe to which the attention is dire£led were fuffered to mix itfelf with the 

 rays admitted into the field of view. In the fame manner it feems that we have little to 

 hope with regard to the improvement of acouftic inftruments for day ufe, if the firft re- 

 quifite, namely, the e.xclufion of unneceffary found, cannot be obtained. 



5. Numerous experiments have fhewn that found can be reflefted, and that the im- 

 preffion on the ear is greater or lefs, according to the difpofition of the refle£ting bodies. 

 Optical inftruments are difpofed in tubes of fuch a length, that the rays of light which 

 arife from a fmall portion of the vifible hemifphere can alone reach the organ of perception. 

 All the others ftrike the furface of the tube, and after one or more reflexions are almoft 

 ■totally abforbed or loft. It remains to be afcertained from reafoning and experiment, 

 how far the fame effeft may be produced with regard to found. With a cylindrical 

 wooden pipe, tliree inches in diameter and eight feet in length, at the diftance of two 

 miles from London, I liftened to the noifes which came from that capital. I think I did 

 not deceive myfelf by any prcpoflcffion, when I diftiniSlly heard the noife and agitation of 

 wheels on the pavement much more ftrongly than any other kind of found. Nearer 

 founds, not in the direction of the tube, were lefs perceived ; and fuch as were loud af- 

 fumcd a mufical tone ; moft probably from the reiterated reflcftions under the fcvcral 

 angles of its reception. I confider this experiment as of little other value than as ferving 

 to convey an hint, that a tube lined with cloth might defend an acouftic inflrument from 

 founds out of the line of its dircclion, while the inftrument itfelf might magnify and 

 render didinCl the found required to be heard. 



6. M. I'crrole conjcftures, at the end of the firft part of his memoir, that fonorous 

 Vol. I. — December 1797. 3 II bodies 



