270 Translation q/'Rey's Essays. 



of water, under which the cook is about to make a large fire. 

 The water will dilate till it runs over the brim : but if the fire 

 be extinguished, it will contract and return to its original bulk. 

 Take this syringe in which the piston is pushed half way down, 

 and the opening'*in front well closed ; push it forcibly, you will 

 reduce the enclosed air to a small compass. Draw back the 

 piston towards you, and though you do not pull it out, yet you 

 will cause the air to dilate to more ample dimensions than it 

 had before. The air being thus compressed, do you doubt that 

 it will have sensible weight in a free air, since it contains more 

 matter in an equal space .'' If the reasons already given in the 

 eighth essay be not suflficient for you, make the experiment. 

 Fill a balloon with air, strongly compressed by means of a pair 

 of bellows, you will find it weighs heavier when full, than empty. 

 And by how much ? By so much as the additional quantity of 

 air in the balloon, weighs, in proportion more than that which is 

 free, under an equal volume. Many have indeed remarked the 

 greater weight of the balloon when full than when empty ; but 

 it has not come within my observation, that any one, hitherto, 

 has known the cause of it. Leaving aside persons of low ac- 

 count, Dr. Scaliger who possesses the true genius of Aristotle, 

 did not understand it ; for in the hundred and twenty-first 

 exercitation against Cardan, he follows the beaten track, 

 holding that pure air is light, and that the balloon gains weight 

 because the air which is next the surface of the earth, such as 

 is forced into the balloon by the bellows, is mixed with vapours, 

 and those little terrestrial bodies clearly discernible in the sun's 

 rays*. But alas ! what good does this mixture do him ? since 

 the experiment is made in an air perfectly similar : certainly it 

 could evince no weight in it unless compressed. If the bal- 

 loon were forcibly filled with the purest air in nature, or even 

 with elementary fire, reason says it still must have weight, if 

 balanced, in the first instance, in the same, and in the second. 



* " Puruin at-rem levem esse. IiiflatO iitrem plenu esse aeris impuri : 

 " sive ab homiiie sufflatus sit : udi cnim multu veLit seed : sive a t'oUe. 

 " Satis enim patet, aerem hac, qui circii terrae est superficiem, vaporibiis 

 " atc|ue terrestribus corpusculis mistum esse : quae in soils radiis apparent 

 " manifest^." Jvl: Caes: Scalig: deSibtil: adHicr: Cardan, L'xetcit : 

 CXXI. p. 181. Lufetiae, M. D. LVII. 



