On the Vibrations which produce Sound. — Echo. if 



In like circumftances the force of found, which is propagated in the atmofphere, depends on 

 the magnitude of the fe^Stion of the air which is at the extremity of the pipe, and the ampUtude 

 of the vibrations of this fedion. It is this furface which ftrikes the atmofphere, and commu- 

 nicates the pulfations*. For this reafon, conical divergent pipes afford a ftronger found than 

 thofe which are cylindrical ; and thefe laft afford a flronger found than pipes which are conically 

 convergent. The firft caufe of the found which a£ls at the mouth end of the pipe would 

 never, of itfelf, excite fuch ftrong pulfations in the atmofphere, as it does excite by the lateral 

 communication in the air contained in a divergent conical pipe. 



The explanation of this phenomenon may be underflood by obferving, i il, That if a number 

 of elaftic bodies be difpofed in progreflion, the firft will imprefs upon the lafl, by the intermedium 

 of the others, more velocity than would be communicated by the immediate flroke. 2. The 

 vibrations excited in the pipe, have a certain permanence which permits them to receive an 

 increafe of force by the united efFeift of fuccellive impulfionsi whereas, in the open atmofphere 

 every pulfation is tranfient and alone. 



Is not the augmentation of found in the fpeaking trumpet, In part owing to the fame caufe 

 of the lateral communication of motion, rather than to the mere refleflion of the fonorous 

 lines from the fides of the tube itfelf? 



I call thofe refonant vibrations, which take place in a tube when found is excited; and I 

 call thofe propagated vibrations, or pulfations, which tranfmit the found through the atmofphere. 

 I have already pointed out a difference, which appears to me to take place between thefe two 

 kinds of vibrations ; namely, that the firfl have a certain permanence and connexion with 

 each other, fo that the latter excites, fupports, and reinforces the former ; whereas, the pulfa- 

 tions which fuccced each other in the atmofphere by the repeated adtion of the refonant body 

 are fmgle, and Independant of each other. 



But the following is a much more remarkable difference between thefe two kinds of vibra- 

 tions.. When at the extremity of a pipe A B C, a refonant vibration Is made in the fe£lion 

 ©fair, BC, fig. 2, Plate VIII, vol. II. experience fhews that this vibration becomes the centre 

 of pulfations propagated all round in P S Q. For on whatever fide the obferver is placed, 

 whether at P or at Q, he will hear the found of the pipe ABC nearly as much as at S. But 

 when there is no pipe, and the vibration at C B is a fimple pulfation propagated through the open 

 air from A to B, in this cafe, the pulfation Is not propagated laterally and completely to P and 

 Q^llke the refonant vibration ; but is contained almoft entirely in the limits B Z and P Y, with 

 adivergence of between 15 and 20 degrees. This fadt has been difputed by various philofophers ; 

 tut it cannot be queflioned, fince It Is well known that we do not hear the echo, or reflected 

 found, from a plain furface, unlefs we place ourfelves In the line of refleflion, or very near it. 

 If the pulfation of the echo were propagated all round, before the reflecting furface, diverging 

 from thence as centre, ought we not to hear the echo in every fituation whatever before that 

 reflecting furface ? We mult therefore admit, with regard to fonorous pulfatlorts propagated 

 in the atmofphere, certain exceptions, and even limits, with regard to the lateral communica- 

 tion of motion which we have pointed out in the firfl propofition, and in the fifth, with regard 

 to water. 



♦ It is known that the material of which a p'pe is made does not perceptibly affefit the found. 



Vol. III.— April 1799. D Mdithn 



