HYDRODYNAMICS. 



471 



OaCapOlary the side of the vessel from which the tube has been 

 moved. The bar // is made to descend by a screw 

 ,cut upon it, till the lower point / touches the water, 



e 



f 

 





Cay Lui- 

 ' 



which is known by the water rising instantaneously 

 round it. When this contact is effected, a small por- 

 tion of the water is removed by the small measure M, 

 at the end of a crooked wire, so that the lower extre- 

 mity I is above the surface of the liquid by a very small 

 quantity. The horizontal wire of the telescope is then 

 made to come into apparent contact with the end t of 

 the bar, and the distance between this and the division 

 corresponding to S, is a true measure of the altitude 

 HS of the fluid. 



It i* obvious from the method now described that 

 the altitude thus found must always be too small, and 

 that a correction depending upon the quantity of wa- 

 ter removed by the measure M should be made. This 

 correction must be very small if the diameter of the 

 claw vessel is great This instrument is obviously 

 liable to a source of error, from the interposition of the 

 glass vesul between the telescope and the fluid column, 

 as the least inequality or difference of parallelism in the 

 parts of the glass opposite to S and H, would produce 

 a sensible error in the result. This, however, may be 

 easily obviated by cutting a piece out of the glass ves- 

 sel, and cementing upon it a plate of parallel glass. 

 We would also recommend, in making very accurate 

 observations, that the sides of the glass tube next the 

 telescope should be either ground flat about S, or 

 have any inequality of refraction removed, by cement- 

 ing upon it a plate of parallel glaw. 



The method of manuring the cohesion of fluids, 

 which we have described for the first tine under CA- 

 PIIHRY ATTRACTION, vol. v. p. 410, coL 2. will, we 

 have no doubt, be found the most correct. The appa- 

 ratus will be greatly improved by using two solid* of 

 the same kind and form instead of one. B v this means 

 the termination of the elevated circle of fluid will be 

 more easily ascertained. 



With the instrument shewn in Fig. 3, M. Gay Lus- 

 sac has made several experiments on the ascent of wa- 

 ter and alcohol in tube* of glass, of which the follow, 

 ing are the results. In these experiments, the tubes 

 ware well wetted with the fluid. 



WATER. 



Height* the Water Tiininatei ll 

 a MilBaistiss *bu*e f tocrm ef UM 

 thctoweMpwniufils Cwtigndc 

 Cwxanty. TlliiMSMSIii. 



Exp. 1. Hi 23.3164 S .:, 



Exp. 2. 



Tubcaui 



i .-.an 

 i.yoaal 



The first of these experiments gives a constant quan- 

 tity of 0.0*6 U in English inches, and the second a 

 constant quantity of 0.04604. We have inserted these 

 in the following table of constant quantities, fur the 

 purpose of giving a general view of the different results 

 which have been obtained. 



Cooataot qi 

 ntj In inches. 



ObOM 

 OOtJ 



OOM 

 oosa 



Newton. See Opfict, p. 366, Sd edit 

 MM. Hauy and Tremery. 

 M JUUirom.' 



Dr Brewter, with a tube 0.0561 of an inch 

 in diameter. 



Constant quan- 

 tity in inches. 



Observers. 



0.0392 Muschenbroek. This is given by Dr T- 



Young as the result of Muschenbroek' s 



best experiment 



0.04O Average assumed by Dr T. Young. 



0.042407 Average of Weitbrecht's seven experiments. 

 0.04641 Gay Lussac, with a tube 1.90 millimetres in 



diameter, and for the lower surface of the 



aniscpa, 



0.04604 Do. with a tube 1 .29 millimetres in diameter. 

 0.048 Benjamin Martin 



0.053 Mr Atwood. 



0.0645 James Bernoulli. 



Our readers will no doubt be much surprised at the 

 great discrepancy among the results in the preceding 

 table, and particularly between those obtained by Dr 

 Brewster and M. Gay Lussac, which were made with 

 instruments founded on the same principle. M. La 

 Place has ascribed these differences to the greater or 

 less degrees of humidity on the sides of the tubes ; and 

 he informs us that, in Gay Lussac's experiments, the 

 tubes were very much wetted. Now, it appears to us, 

 that though by this method the water will always stand 

 nearly at the same height in the same tube, yet it does 

 not afford us an accurate measure of the height of ascent. 

 Let us suppose jhat a tube r l y of an inch in diameter is 

 so perfectly cleared of all grease nd extraneous matter, 

 that the attractive force of the glass is allowed to exert 

 itself without any diminution, and that the water stands 

 at the height of 3.3 inches. Let us now suppose that by 

 some means or other a film of water of the thickness 

 of 7 4? f ln inch is introduced at the upper mil 

 of the tube, and equably diffused over its interior 

 surface, it is obvious that the water will rise above in 

 former level, and consequently to a height greater than 

 that which is due to the force exerted by the tube and 

 tin- mutual action of the fluid. We conceive, therefore, 

 that M. Gay Lussac's measures err in excess ; and that 

 the error increases inversely as the diameter of the tube. 



The discrepancies in the table appear to us also to 

 be partly owing to the different kin Js of glass employ. 

 ' I 1'he flint glass, of which tubes are composed, va- 

 ries very much in its density ; and there is every reason 

 to believe, both from analogy and from some direct ex- 

 periments which we have made, that the substance of 

 the tube has a very perceptible influence on the height 

 to which the fluid ascends. 



The following were the results which M. Gay Lui- 

 sac obtained for alcohol. 



ALCOHOL. 



On Capillary 

 Attraction 



and the 



Cohesion of 



Fluid*. 



The results of experiments 1 and 2, when reduced to 

 English inches, give .01798 and .01840 for the value of 

 the constant quantity. The constant quantity for AU 



found that water rose 11.7 Swedish lints in tube 0.7 ef line in diameter. f This is a nwan of 5 eipcrimenU. 



