WITHDRAWN FROM THE ACTION OF GRAVITY. 629 



points and the eye must pi'oduce some effects of refraction, 

 which would introduce a slight error into the value obtained. 

 To avoid this inconvenience, we need only expose the rings, by 

 allowing the liquids to escape from the vessel by the stop-cock 

 (note 2 to § 9), then remove the minute portions of the liquid 

 which remain adherent to the rings by passing lightly over their 

 surface a small strip of paper, which must be introduced into 

 the vessel through the second aperture. The drops of alcoholic 

 liquid remaining attached to the inner surface of the anterior 

 side of the vessel must also be absorbed in the same manner. 

 In the second place, as it would be difficult for the rings to be 

 rigorously parallel, their distance must be measured from two 

 opposite sides of the system, and the mean of the two values 

 thus found taken. The following are the results which I ob- 

 tained. The mensuration of the distance between the summits 

 gave first, in four successive operations, the values 'J6''J'J, 76*80, 

 76*85 and 76*75 millims., the mean of which is 76*79 millims. 

 But after the alcoholic liquid had been again agitated for 

 some time, to render its homogeneity more certain, two new 

 measurements taken immediately afterwards gave 77*05 and 

 and 77'00 millims., or a mean of 77'02 millims. The distance 

 betw^een the external planes of the rings was found, on the one 

 hand, by two observations, which agreed exactly, to be 57*73 

 millims ; on the other hand, two observations furnished the 

 values 57*87 and 57*85 millims., or as the mean 57*86 millims. 

 Taking then the mean of these two results, we get 57*79 millims. 

 as the value of the distance between the centres of the external 

 planes. Hence, if we assume the first of the two values ob- 

 tained for the distance of the summits, 76*79 millims., we find 



7^-/= ^^'^^ 7^^'^^ =9*50 millims.; 



and if from the second result, 77*02 millims., we find 



^ 77*02-57*79 ^^.^^ miUims. 

 •^ 2 



These two elevations evidently differ but little from 9*41 

 millims., the altitude deduced from theory (§ 40) ; in the first 

 case the difference does not amount to the yi^dth part of 

 this theoretical value, and in the second it hardly exceeds 

 ygjjdths. These differences undoubtedly arise from slight re- 



