EFFECT OF SOUND ON THE BAROMETER. 183 



the height of the mercury about 5 thoufandths lower than my- 

 ielf, which brings our obfervations to a very near agreement. 

 The following obfervations prove this : 



On the top of the tower, Mr. P. - «■ 29.424 



Ditto, by me 29.430 



At the foot of the tower, Mr. P. - 29.639 



Ditto, by me - - - - - 29.642 



In the court of the Englifh Nuns, by Mr. P. 29.676 



Ditto, by me 29.682 



And I mould think that the difference of eyes may frequently 

 caufe fuch a variation among different obfervers; at leaft in 

 delicate obfervations, it will be always prudent to make the 

 experiment. 



NOTE BY DR. YOUNG. 



THESE obfervations appear to agree too well with each Obfervations by 

 other, to allow us to doubt of their accuracy. It therefore be- Dr * Youn 6* 

 comes neceffary to inquire into the caufes of the different 

 heights of the barometer. It is indeed barely poffible, that a 

 fudden ftroke of the clapper on the bell might produce a 

 greater agitation of the building than the preceding alternate 

 motion of the bell itfelf: but this explanation cannot be called 

 fatisfaclory. It is certain, that there was neither more nor lefs 

 air in the tower while the bell was founding, than while it was 

 filent; the mean denfity of the air could therefore not have The mean den* 

 been changed; and if the alternate motions of the particles of flty of theair 

 air which constitute found, had taken place by equal degrees, changed, 

 and with equal velocities, in each oppofite direction, there is 

 no reafon to fuppofe that the increafe of prefTure on the furface 

 of the mercury, at one inftant, could have tended to raife it, 

 more than the decreafe of preffure, in the oppofite irate of the 

 undulation, would have depreffed it. But the fame confe- If the motion of 

 quence does not follow, if we conceive the motion of the air, more ra Tddian 

 in advancing, to be more rapid, but of fhorter continuance, of return in the 

 than its retrograde motion. For if the wind blew for one hour undu | atl0ns > » 



. ,. , i r • i-i r w °u!d have the 



with a velocity ot 4, and the lame air returned, in the courfe effect of a pref- 

 of two hours, with a velocity of 2, an obftacle upon which it fure « 

 had acted in both directions would not be found in its original • 



place; for the action of the wind upon an obftacle is as the 



fquare 



