MOLECULAR PHYSICS. 87 



II. INSTRUMENTS CONNECTED WITH FLUIDS. 



IN countries where the fertility of the soil is capable of being 

 greatly increased by artificial irrigation, the attention of all ingenious 

 persons is naturally directed to devising means whereby the labour 

 of raising water may be diminished. Hence we find that in China 

 and in India, but especially in Egypt, great progress was made 

 in the art of producing and guiding the motion of water. 



The first pump worked by a piston of which we have any 

 account seems to be that invented by Ctesibius of Alexandria, 

 about 130 B.C. 



The construction of this pump, as described by Vitruvius, 

 resembles that of the modem fire-engine. It had two barrels, 

 which discharged the water alternately into a closed vessel, the 

 upper part of which contained air. This air-chamber acted as a 

 reservoir of energy, and equalised the pressure under which the 

 water was emitted from the discharge pipe. 



Hero, a scholar of Ctesibius, invented a number of ingenious 

 machines. He delighted in curious combinations of siphons, by 

 which fountains were made to play under unusual circumstances. 

 We should call such machines toys, but though to us they have 

 no longer any scientific value, we must regard them as among the 

 first instances of apparatus constructed not in order to minister 

 directly to man's necessities or luxuries, but to excite or to satisfy 

 his curiosity with respect to the more unusual phenomena of nature. 



From the time of Ctesibius and Hero to that of Galileo (1600) 

 pumps were constructed chiefly for useful, as distinguished from 

 scientific, purposes, and considerable skill was developed in the 

 art of forming the barrel and piston so as to work with a certain 

 degree of accuracy. 



Galileo showed that the reason why water ascends in a sucking 

 pump is not that Nature abhors a vacuum, but that the pressure 

 of the atmosphere acts on the free surface of the water, and that 

 this pressure will force the water only to a height of 34 feet ; for 



