PNEUMATICS. 351 



suggested. That some of the ancients had formed certain vague ideas 

 of the gravitating power of the air, is obvious from many of their 

 works still extant ; but their notions were very confused, and involved 

 in considerable obscurity. Thus Aristotle says, that all the elements Aristotle. 

 have weight, with the exception of fire; adding, that a bladder inflated 

 with air will weigh more than when it is quite empty. (De Caelo, 

 lib. iv. c. i. op. torn. i. p. 485.) Plutarch and Stobasus quote 

 Aristotle as teaching that the weight of air is between that of fire and 

 earth; and the latter philosopher himself quotes Empedocles as 

 attributing the act of respiration to the pressure of the air, by which it 

 insinuates itself into the lungs. Again, Plutarch (De Placit., lib. iv. 

 c. xxii. torn. ii. p. 903) expresses, in similar terms, the opinion of 

 Asclepiades on this subject, and represents him as saying, that the 

 external air, by its weight, opened its way by force into the breast. 

 Hero of Alexandria, in his work ' Spiritalia,' applies the principle of Ctesibiusand 

 the elasticity of the air to produce and explain various effects, in such Hero> 

 a way, as sufficiently to convince us that he was no stranger to several 

 of the properties of atmospheric air ; and Ctesibius, adopting the prin- 

 ciple of its elasticity, constructed wind-guns, which afterwards passed 

 for modern inventions. There is, however, some difference between 

 the ancient and modern air-gun : in that of Ctesibius, for example, the 

 ball was not immediately exposed to the action of the air, but was 

 impelled by the longer arm of a lever, while the air acted on the 

 shorter ; but the principle of operation is the same in both, and shows 

 clearly that the elastic property of common air, if it could not be accu- 

 rately measured, was at least known at that period. To this philo- 

 sopher is also commonly attributed the invention of the sucking-pump. 



Hero, to whom we have above referred, was a contemporary and 

 scholar of Ctesibius ; he describes in his treatise ' On Pneumatics,' a 

 number of very ingenious inventions, a few of which are calculated for 

 utility, but the greater part only for amusement ; they are principally 

 siphons, variously concealed and combined, fountains and water-organs, 

 besides the syringe and fire-engine. This machine is said to agree in 

 principle with the common engine of the present day ; it consists of 

 two barrels, discharging the water alternately into an air-vessel ; and 

 it appears, from Vitruvius, that this was the original form in which 

 Ctesibius invented the pump. Hero supposes the possibility of a 

 vacuum in the intervals of the particles of a body, observing, that 

 without it no substance could be compressible ; but he imagined that 

 a vacuum could not have existed throughout a perceptible space, and 

 thence derives the principle of suction. The air contained in a given 

 cavity, he says, may be rarefied by sucking out a part of it, and he 

 describes a cupping instrument, which approaches very nearly to an 

 imperfect air-pump. 



It appears, therefore, that at this time, viz. (B. c. 100), many of the 

 properties of air were fully understood, particularly its gravity and 

 elasticity; but the followers of these philosophers, abandoning the 



