88 SCIENTIFIC APPARATUS. 



when this height is reached, the water rises no further, and this 

 shows either that the pressure of the atmosphere is balanced by 

 that of the column of water, or else that Nature has become 

 reconciled to a vacuum. 



From this time the production of a vacuum became the scienti- 

 fic object aimed at by a great number of inventors. Torricelli, 

 the pupil of Galileo, in 1642, made the greatest step in this direc- 

 tion, by filling with mercury a tube closed at one end and then 

 inverting the tube with its open end in a vessel of mercury. If 

 the tube was long enough the mercury fell, leaving an empty space 

 at the top of the tube. The vacuum obtained by filling a vessel 

 with mercury and removing the mercury without admitting any 

 other matter is hence called the Torricellian vacuum. Torricelli, 

 therefore, gave us at once the mercury-pump and the barometer, 

 though the subsequent 'history of these two instruments is very 

 different. 



A little before 1654 Otto Von Guericke, of Magdeburg, first 

 applied the principle of the common pump to the production of a 

 vacuum. The fitting of the pistons of the pumps of those days, 

 however, though sufficiently water-tight, was by no means air-tight. 

 He therefore began by filling a vessel with water and then 

 removing the water by a water-pump. In his experiments he met 

 with many failures, but he continued to improve his apparatus till 

 he could not only exhibit most of the phenomena now shown in 

 exhausted receivers, but till he had discovered the reason of the 

 imperfection of the vacuum when water was used to keep the pump 

 air-tight. 



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

 whose laboratory he was an assistant ; it was more convenient 

 than Guericke's, but the vacuum was not so perfect : yet Boyle's 

 numerous and judicious experiments 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 



