224 FERGUSON'S LECTURES. 



LECT. resist its pressure upward, the lead is thereby pressed 

 ,^-v'"^' upward, contrary to its natural tendency by gravity. 

 If the syringe, so loaded, be hung in a receiver, and 

 the air be exhausted, the syringe and lead will descend 

 upon the piston-rod by their natural gravity ; and, 

 upon admitting the air into the receiver, they will be 

 driven upward again, until the piston be at the very 

 bottom of the syringe. 



27. Let a large piece of cork be suspended by a 

 thread at one end of a balance, and counterpoised by a 

 leaden weight, suspended in the same manner at the 

 other. Let this balance be hung to the inside of the 

 top of a large receiver ; which being set on the pump, 

 and the air exhausted, the cork will preponderate, and 

 shew itself to be heavier than the lead ; but upon letting 

 in the air again, the equilibrium will be restored. The 

 reason of this is, that since the air is a fluid, and all 

 bodies lose as much of their absolute weight in it, as 

 is equal to the weight of the bulk of the fluid, the cork 

 being the larger body, loses more of its real weight 

 than the lead does ; and therefore must in fact be 

 heavier, to balance it under the disadvantage of losing 

 some of its weight ; which disadvantage being taken oft' 

 by removing the air, the bodies then gravitate according 

 to their real quantities of matter, and the cork which, 

 balanced the lead in air, shews itself to be heavier when 

 in vacuo* 



28. Set a lighted candle upon the pump, and cover 

 it with a tall receiver. If the receiver holds a gallon, 

 the candle will burn a minute ; and then, after having 

 gradually decayed from the first instant, it will go out : 

 which shews, that a constant supply of fresh air is ne- 



Note 60. This experiment may be adduced as an answer to the com- 

 mon query " Which is heaviest, a pound of lead, or a pound of fea- 

 thers ?" and a reference to the fact of the air's buoyancy will shew 

 that the larger body, or the one whose specific gravity is least, must 

 of necessity have the greatest portion of its weight subtracted. 



