452 fbejucking [Book VII, 



ihouid not exceed that length. In common practice 

 it is indeed feldom made fo long as thirty-two feet. 

 In order that the preffure of the atmofphere may be 

 fuch as to fuftain a column of water of that height, 

 it is neceffary, firft, that the fucking purnp ihouid be 

 made with the greateft exactnefs, and that it mould 

 continue in 'a perfect ftatei fecondly, that it mould be 

 placed upon the level of the fea, or very near it, be- 

 caufe it is there that the preflure of the atmofphere 

 is rnoft forcible j thirdly, that the preffure of the air 

 ihouid not vary ; but in general all thefe circumftances 

 are not required. Mechanics are fatisfied, in general, 

 with making the afpiration pipe twenty-three or twenty- 

 four feet long. If it is neceffary to elevate water to 

 a greater height, the forcing pump is more commo- 

 dious for that purpofe. The latter, it is true, is fub- 

 jedt to feveral inconveniences. It is neceffary to place 

 the body of the pump either in wells or in a bafon.; 

 and when it is required to repair it, which often hap- 

 pens, one of two things is neceffary, either to empty 

 the well or the bafon, or to draw up the body of the 

 pump, which is extremely troublefome and expenfive. 

 To remedy thefe inconveniencies, the beft and moft 

 iifual mode is, when water is to be elevated from a 

 great depth, to make the pump at once both a forcing 

 and fucking pump, in the manner which I mall pre- 

 fently explain. 



In the year 1766 it was reported, through the chan- 

 nel of the public papers, that at Seville, in Spain, a 

 fimple fucking pump had been conflructed, which ele- 

 vated water to the height of fixty feet, and it was con- 

 fequentlv concluded, that the world had till then been 

 notorioufly impofed upon by the current maxim, that 

 the preffure of the air could raife a column of water 



only 



