278 LECTURE XXX. 



quadruple height was required in order to produce a double velocity ; and 

 his assertions were afterwards fully confirmed by Mariotte and by 

 Guglielmini.* j 



A little before the year 1654, 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 water 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 inter- 

 vening 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, w T hen 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 expe- 

 riments 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 imperfection in the vacuum. An 

 account of his experiments was first published in different works, by 

 Caspar Schott,t and afterwards by himself, in his book entitled Experimenta 

 nova Magdeburgica, printed in 1672 at Amsterdam. 



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

 ratory he was an assistant : it was more convenient than Guericke' s, but 

 the vacuum was not so perfect ; yet Boyle's numerous and judicious expe- 

 riments gave to the exhausted receiver of the air pump the name of the 

 Boylean vacuum, by which it was long known in the greatest part of 

 Europe. Hooke' s air pump had two barrels, and with some improvements 

 by Hauksbee,^ it remained in common use until the introduction of Smea- 

 ton's pump, which, however, has not wholly superseded it. The theory of 

 pneumatics was also considerably indebted to Hooke's important experi- 

 ments on the elasticity of the air, which were afterwards confirmed and ex- 

 tended by Mariotte and Amontons, in France, by Hales in this country, 

 and by Richmann at Petersburg. 



About the same time the first steam engine was constructed by the cele- 

 brated Marquis of Worcester. Hints of the possibility of such a machine 

 had been given a hundred years before, by Matthesius, in a collection of 

 sermons entitled Sarepta, and at a subsequent period by Brunau;|| but the 



* See authorities in Lect. XXIV. 



f Magia Universalis, 4vols.4to, Wurtzb. 1657. Mechanica Hydraulico-pneu- 

 matica, 4to, 1657. Technica Curiosa, 4to, Norimbergee, 1664. 



J Hauksbee, Physico-Mechanical Experiments, 4to, Lond. 1709, p. 1. 



Kepler in Bergmannische's Journal, 1791, ii. 263. 



|| Hints towards a Steam Engine, in 1627, Nich. Jour. vii. 311. 



