188 Mr. J. E. H. Gordon on the Effect of Variation of 



that the pressure of the air in the tube should equal that of the 

 external atmosphere, air bubbled through the acid as long as 

 the difference of pressure inside and outside the tube exceeded 

 that of the inch of acid which had to be displaced, and then 

 the tap was opened direct to the outside air. The external 

 diameters of the tubes were about 2*94 and 2*76 inches respec- 

 tively, and the diameters of the balls *94 and *92 inch. 



The Experiments. 



In the experiments which are the subject of the present 

 paper, one of the tubes (A) was left open to the atmosphere, 

 and its discharging point placed at a standard distance either 

 6, 8, or 10 inches from the ball ; and the other tube (B) being 

 nearly exhausted, experiments were commenced at the low 

 pressure, and then a little air was let in between each observa- 

 tion. The tubes were so connected to the coil that the dis- 

 charge would pass in whichever tube offered least resistance. 

 The discharging-distance in B was then varied and adjusted to 

 the shortest distance, which caused the whole discharge to pass 

 in A. The distance between the points of B being noted, the 

 points were then brought nearer together till they reached the 

 longest distance at which the whole discharge passed in B. 

 The mean of these two distances was taken as the distance 

 which, at the pressure then being worked with, interposed in B 

 a resistance equal to that of the standard length in A of air at 

 the pressure of the atmosphere. 



Let us call this mean " mean B spark." Now, if the law 

 that the spark is inversely proportional to the pressure holds, 

 we should have for the same series of experiments, 



{mean B spark} {pressure in B} = const. ; 



and to compare different sets made with different distances in 

 A and with the barometer at different heights, we should have 



{ mean B spark } { pressure in B } 



— const. 



{distance in A} {height of barometer} 



If the two tubes and the discharging points were precisely 

 alike, this constant would be unity. Any slight difference in 

 the shape of the points and balls, however, would cause it to 

 differ from unity, but would not affect its constancy. 



The Table (pp. 190, 191), which explains itself, gives the 

 results of several sets of experiments arranged in ascending 

 order of pressures. 



The results which I deduce from it are : — 



(1) From a pressure of about 11 inches up to that of the 



