30 Dr. Mills on the Ascent of Hollow 



Table V. 

 t. p. tp*. 



15-5 -00438 -00029735 



31-0 -00310 -00029791 



60-0 -00223 -00029844 



97-0 -00177 -00029705 



146-2 -00143 -00029806 



Equation, tp 2 = c= '00029776. 

 Probable error of c= '000000169 = 0'06 per cent. 

 The product of the square of the unbalanced unit pressure 

 into the time is therefore a practically constant quantity. 



III. Change of Content. 



9. In all the experiments above recorded the bulb contained 

 air, probably at a pressure slightly below that of the atmo- 

 sphere. The mean density of (bulb + contents + weight) is 

 of course affected by the contents of the bulb, and then also 

 the rate of ascent. 



If we regard as correct the mathematical relation last 

 arrived at (8), it is easy to deduce from it an equation which 

 involves the specific gravity of the contained gas. Assuming 

 other conditions to remain unchanged, the density of the gas 

 is 



7 =A + B^/i=^. 



In this expression t is the time of ascent through the con- 

 stant height for any particular gas, 6 the time of ascent cor- 

 responding to a gas of density A, and B the density of a 

 gaseous content which just fails to let the bulb sink. It is 

 convenient to put A = 0, which corresponds to the bulb being 

 vacuous : B and 6 are then deduced, as the " constants of the 

 bulb," from two independent determinations. 



10. In order to change at pleasure the contents of the bulb 

 its form was slightly altered, as shown in fig. 2, and 

 its volume enlarged. The two stoppers with which Fig. 2. 

 it was furnished were coated, on their ground sur- 

 faces, with a scarcely perceptible film of grease; Jul 

 the operator's hands were encased in gloves ; and ( j 

 every experiment was carried out as speedily as pos- 

 sible. The gases were employed in the dry state, at 

 an average barometric pressure of about 75*7 cen- 

 tim.; in the following table their densities are referred 

 to hydrogen. Each result is a mean of several de- 

 terminations; and the times are given in thirds of 

 seconds. i 



