70 APPARATUS FOR CONDUCTING SOUND. 



mechanics, optics, and acouftics, which are now exhibiting 

 for amufement, may hereafter become ufeful in the common 

 affairs of men, is a matter on which we are not at prefent able 

 to judge. 



Phantafms by The daggers and death-heads which are made to appear in 

 the camera ob- ., . , vr 



fcura behind a tne air * b Y me ans ot a concave mirror ; and the ghofts and 

 fcreen. goblins, that are conjured up by means of the magic lantern 



of Philipfthal, not only ferve as amufement, but they may con- 

 tribute to check the growth of fuperftition, by (hewing in an 

 agreeable manner how eafy it is to impofe upon the fenfes. 

 Speaking ma- Another way of deceiving the fenfes, is by (peaking ma. 

 chines. Thefe are of very ancient date, and have been (o 

 much improved by the moderns, as to attraft much attention. 



?rfoVta!he 0maS N ° Ionger ag ° than the rei S n of Charles H. one Thomas Irfon 

 t7th century, excited much wonder in the king and his whole court, by a 

 machine of this kind *. It is now well known that the found 

 was conveyed to the mouth of the ftatue by means of tubes 

 artfully concealed ; but the principle on which the fpeaking 

 machines are now conftru&ed, appears to be at this time very 

 little underftood. 



rt U tSroS°h d tim- lt WaS known in the da ? s of Plin y> that if a long beam of 

 bcr. timber received a flight ftroke at one end, the found was dif- 



tin£tly heard by a perfon whofe ear was applied to the other 



end, though it could not be heard at the fame diftance through 



the air. 

 This property From the following experiments which I made on this pro- 

 rpeakingappa- P ert y °f wood, it appears that acouftic inftruments may be 

 ratus. conftrucled for converfing at a diftance, without the affiflance 



of tubes to convey the found. 



EXPERIMENT I. 



A deal rod 16 I took a deal rod 16 feet long, and about an inch fquare, 



contaft made the anc * a ^ ter navm g 6 xe d one en( * °f & into the fmall end of a 

 medium be- fpeaking trumpet, I laid it upon two fupporters or props in 

 tween two an horizontal politiort. One of the props was placed under 



the trumpet, about three inches from its wide end, and the 

 other prop was placed near the other end of the rod. An- 

 other fpeaking trumpet was then laid acrofs the rod, about 

 three inches from the end. The wide end of this trumpet 



• Beckmann's Hiftory of Inventions, III. 334. 



refted 



