of Small Gaseous Pressures. 421 



error of only about a fifth of one per cent, of the quantity 

 measured. 



To show how small is the effect of variable capillary depres- 

 sion in the large mercury columns, the following measurements 

 were made July 25, lb97. No correction was made of acci- 

 dental capillary differences, but the columns were always 

 observed with a falling meniscus. The zero of the micrometer 

 was freshly adjusted for each reading ; and before each of the 

 six sets of readings the mercury was lowered, and then 

 readjusted to the proper height in the gauge-head. 



M. 



M. 



M. 



M. 



M. 



M. 



2-210 



2-203 



2-209 



2-198 



2-198 



2202 



•204 



•195 



•202 



•203 



•204 



•198 



•209 



•198 



•204 



•208 



•200 



•196 



•203 



•204 



•210 



•200 



•196 



•208 



•203 



•192 



•202 



•198 



•196 



•203 



Means 2-2058 2-1984 2 2054 2-2014 2-1988 2-2014 



Calculating the probable errors, we have : 



Six mean readings...... 2-20187 M. ±0-00073 M. 



All readings 2-20187 „ +0-00059 „ 



The effect of not equalizing the capillary depression is very 

 apparent when these results are compared with the earlier 

 ones quoted. But on account of increased skillfuiness of 

 observation, due to long experience, the individual readings 

 of each set are more uniform than before ; so that the net 

 result is better. 



In this example we have the measurement of about two 

 millionths of atmospheric pressure, with a probable error of 

 only one part in three thousand of the quantity measured. 



From the foregoing we may safely conclude that with the 

 apparatus described, small gaseous pressures may be easilv 

 measured with a probable error of less than a thousandth pait 

 of a millionth of atmospheric pressure. 



The smallness of this fraction is difficult to realize. It is 

 comparable with a thousandth part of a milligram, in a 

 thousand grams ; or a single kernel of wheat in two thousand 

 bushels ; or an inch and a half in the circumference of the 

 earth ; or the thickness of a sheet of tissue paper in 16 miles. 



