OK THE HISTORY Of HYDRAULICS AND PNEUMATICS. 535 



difterent distances below the surface of the water in the reservoir. Castelli's 

 experiments, made about \6iO, were the first of the kind, and some of 

 them really tended to the improvement of the science of hydraulics, 

 but others appeared to show, that a double height of the head of water 

 produced a double discharge. Torricelli's more accurate observations 

 proved that a quadruple height was required in order to produce a double 

 velocity; and his assertions were afterwards fully confirmed by Mariotteand 

 by Gughelmini. 



A little before the year l65i, Otto von Guericke, of Magdeburg, first 

 constructed a machine similar to the air pump, by inserting the barrel of a fire 

 engine into a cask of water, so that when the M'ater was drawn out by the 

 operation of the piston, the cavity of the cask remained nearly void of all 

 material substance. But finding that the air rushed in between or through 

 the staves of the cask, he inclosed a smaller cask in a larger one, and made 

 the vacuum in the internal one more complete, while the intervening space 

 remained filled with water; yet still he found that the water was forced into 

 the inner cask through the pores of the wood. He then procured a sphere of 

 copper, about two feet in diameter, and was exhausting it in the same way, 

 when the pressure of the air crushed it, with a loud noise. This machine 

 was more properly a water pump, than an air pump, but the inventor soon 

 after improved his apparatus, and made all the experiments which are to this 

 day the most usually exhibited with the air pump, such as the apparent 

 cohesion of two exhausted hemispheres, the playing of a jet by means of the 

 expansion of a quantity of air inclosed in a jar, the determination of the 

 air's weight, and others of a similar nature. He also observed, that for 

 very accurate experiments; the valve of the pump might be raised at each 

 stroke by external force; and he particularly noticed the perpetual production 

 of air, from the water that he generally employed, which caused an imper- 

 fection in the vacuum. An account of his experiments was first published 

 in different works, by Caspar Schott, and afterwards by himself, in his 

 book intitled Experimenta nova Magdeburgica, printed in 1672 at Am- 

 sterdam. 



In the year 1658, Hooke finished an air pump for Boyle, in whose la- 



