On Capillary AB'ion, 199 



be {l»o\vn, is, like the capillary aftion itfelf, inverfely as the 

 width of the bore. The two oppofite eft'e<:ls, being thus pro- 

 portional, are confounded, and their difference only is ob- 

 ferved. In the cafe of giafs, the capillary aftion greatly pre- 

 ponderates : one confequence of this is the concavity re^ 

 marked at the top of the column; a clear proof that the lateral 

 exceeds the central adhefion. If the upper larface were per- 

 feftly flat, ihofe two forces would be in exa6i: eqailibrium. 

 But the convexity of the top of the column prove,-; that the 

 fluid has a greater altradfion to its own particics than to glafs. 

 Such i.-; the cafe with quickfilver, which, inftead of afcending 

 in capillary tubes, fuflers a depreffion' accof^ing to the fam6 

 law. This remarkable fail has been afi'iibed to fome re- 

 puluon exifting between quickfilver and glais. But I resrard 

 fuch a fuppofition as equally unneceflary and improbable. 

 The fame confequence would, wo doubt, take place with 

 water in polifhed tubes of fteel or brafs. If I place a drop of 

 water on a fniooth furface of metal, or a globule of quick- 

 filver on a plate of glafs, neither of them will feem at all af- 

 fected by the contail, but will obey the altraAlon of their 

 own panicles, and therefore afll-A the fj)herical form. 



Hence the mercury in a barometer can never mount to its 

 true height; and the error will be the more confidcrable in 

 proportion as the tube is narrow. If two tubes of ditiercnt 

 diameters be carefully filled and planted in the fame bafin, 

 the mercurial column will ftand vifibly higher in the wider 

 tube. This aberration has been generally neglected, thouoh 

 it muft evidently atl'edt the barometric calculation of the alti- 

 tudes of mountains *. 



On the fame principle I would explain the familiar experi- 

 ment of a needle fwimming on the furfacc of water. It is 

 fuperfluous to have recourle to any fuppofed repullion. The 

 internal cohefion of the water opj)ofes its divilion ; a gentle 

 cavity is formed, which, prefiing upwards, fupports llu- needle. 



Above twenty years ago, that able chemill ]\I. Guyton- 

 Morveau, examining into the nature of ehemieal aflinilics, at- 

 tempted to determine the relative attrad-tion of a variety of fiib- 

 Itances, from the forte required to detach linall plates of glafs 

 or metal from their contact with water or quickfilver. The 



• Suppofc the iTicrcurial column at tlic bottom ot the moiintain to be 

 50 inchcii, at the fiinuiiit 15 ; and kt the licprciiion he L'()U;il tu rlic loth 

 of an ir.cli, which coiTtlponds to irdiii.iry tubes. Then lUc tiitrecltd co- 

 Jumni. nre 30"05 and iS'Oj; but the r.iiio of thel'c numhtrs \% ditftrcnt 

 fion) thiit of 30 to ii;, and ciinU<|ucntiy it will j^ivc a different altitude. 

 The conimon calciihiion will reprefent the inounuin above 90 I'cet higher 

 tliin it ouj^ht to be. 



N 4 idea. 



