190 N^W MERCURIAL AIR-PUMP. 



Air pump oper- of the column M N, which in every fiicceffive flroke becomes 

 cu"^i/a"wood- ^'8^*^'"» ^''' ^^^ exhauftion is nearly coinpleat, when its height 

 rn barrel, &c. will be that of the barometer. 



In order to equalize the preffing force more nearly during the 

 whole operation, the upper end of the pilton may be loaded 

 with a weight equal to half the original prefliire. 



The mercury being a fluid, fills every angular fpace as it 

 pafles, till it opens its way through the valve R; confequently 

 no particle of air can be left in a condenfed ftate to be more 

 rarified or diminillied by the next ftroke of the pifton. By 

 this means the common defefl of air-pumps feeras to be 

 remedied, and the exhauftion unlimited. Belides, as neither 

 moifture or oil are introduced in this pump, the vapours arifing 

 from them in great exhauftions are prevented. 



The quantity of mercury required, when the diameter of 

 the wooden box and of the fphere P are four inches, will be 

 about fixty pounds, which if taken out by the cock O, may 

 ferve alfo for another ufeful apparatus in the fame laboratory: 

 the mercurial air holder. 



Several projects for mercurial air-pumps have been publifhed 

 before, although I think none executed; but a comparifon 

 will, as I fuppofe, ealily ftiew this conftrudion to be quite 

 different from any of them. 



For greater foUdity, I have in the execution made three- 

 fourths of the tube M P of iron. The fphere P is hkewife 

 fixed and faftened by means not fliewn in the figure, but which 

 may without dilTiculty be apprehended. 



Excepting the cock Q R, which requires fome precifion of 

 workmanfliip, all the parts of the machine may be executed by 

 common workmen, fucb as carpenters, fraiths, &c. who ar«i^ 

 every where to be found. 



VI. On; 



