a!r blown from bellows. ij?^. 



iiit contact with it in the latter than in the former cafe. 



Thefe remarks fupport and explain the fa£ls noticed by 



Dr. Black and Boeiliaave. Agitation of the air is merely 



fuppofed, and not that it (hall be either condenfed or rarified. 



Many fafts concur to (hew, 'that the capacities of elaflic ^Ir is heated 



fluids for heat are encreafed by rarefa<Slion, and diminifhed 



by condenfation ; proofs of which we have by experiments 



in the air-pump and condenfer, and in (he late experiments 



of explofions produced in the chamber of the condenfing 



fyringe. If we attend to this law, we muft infer that the 



air in a pair of bellows, being fuddenly compretTed by a force 



perhaps equal to one twentieth of an atraofphere or more^ 



will acquire an increafe of temperature ; and if in this dif- 



pofilion to give out heat, it be made to rufli againft the ball 



of a thermometer, it will heat the mercury, and caufe it to 



rife in (he tube. Now, in order to reconcile both the Whence the 



refuUs of Mr. Winter, and of Boerhaave to truth, we mu(l [^^5 ^-,11 ^ejiot^ 



recollefl that bellows, like the unfortunate traveller in Efop's and the remoter 



Fables, can blow hot and cold at the fame time. If the ther- >? '2:1"'^ ^^ 



_ Its motion. 



mometer be held very near tl)e aperture, the warm air will 

 heat the mercury; but if it be held at a greater diflatice, 

 where the warm air has become plentifully mixed with cold, 

 the eifecl of its temperature may be altogether inconfiderable, 

 while that of the agitation continues to be efFe£lIve: that is. 

 to fay, the thermometer if already at the common temperature,. 

 will neither rife nor fall; if it be already hot the fleam will 

 cool it ; or if cool the fteam will heat it, 7'hus it is, to return 

 to our Iravelier, that we breathe upon our fingers held clofe 

 to our mouth when we mean to warm them ; but when we 

 widi to produce cold, we hold the fubjcd at a diftance, and 

 blow at it. 



As the^hermometer falls in tlie pneumatic vacuum, I fup" 

 pofe thftre may be fome miflake in the poft^cT;p^ 



Vol. XIIL— FtBKUAKTi;, ISOt^, O Jteoa^rt 



