of Small Gaseous Pressures. 419 



precautions are necessary. After filling A and B with mercury, 

 time must bo allowed for the compressed gas to cool. The 

 effect of changing barometric pressure is nearly eliminated 

 by so regulating the quantity of mercury in F that its surface 

 is in the small tube at the bottom of the cistern, when the 

 gauge is properly filled. Its area is then very small, as com- 

 pared with that in B and C. The height of the meniscus in 

 both tubes is easily adjusted sensibly equal by a little mani- 

 pulation. I always raise the mercury above the point at which 

 readings are to be taken, and then lower it, so as to read on a 

 falling meniscus. This is highly important. 



Some trouble was occasionally experienced at first from 

 electrostatic induction between the mercury in B and the 

 glass above it. This was shown by distortion of the meniscus 

 when it was brought very near the glass. The difficulty was 

 partially, but not wholly remedied by putting mercury in the 

 outside open end of the gauge-head, and connecting it by a 

 flexible conductor with the mercury in the cistern F. A 

 complete remedy was effected by moistening the inside of the 

 gauge-head with a dilute solution of phosphorus pentoxide. 

 This became completely dried by the anhydrous phosphorus 

 pentoxide in N, but was of course not dehydrated ; and hence 

 always remains conducting, and dissipates the static charge. 



Large pressures, up to a thousand millionths or more, are 

 readily measured with this apparatus by finding with the 

 cathetometer the distance between the mercury in B and the 

 end of the head above it ; from this is quickly calculated 

 the necessary multiplier for the number of millimetres differ- 

 ence in height between the columns in B and C, also measured 

 by the cathetometer, in order to express the result in millionths. 

 For very small pressures, the micrometer wires are set at such 

 a distance apart as to give a convenient constant (usually 2); 

 and the column in B is adjusted this distance away from the 

 glass ; careful allowance being made for the thickness of the 

 wires. Then the micrometer is used for repeated measure- 

 ments of the difference in height of the mercury in B and C. 

 The disturbing effect of bias is entirely eliminated by giving 

 the micrometer screw a partial turn after each reading. Thus 

 the next measurement is made without any knowledge of its 

 difference from the preceding one, until the eye is removed 

 from the telescope. 



In my earl\ experience with the apparatus, unusually care- 

 ful measurements of very small pressures were often made to 

 determine how far its indications might be relied upon. In 

 this connexion I quote as follows from my notes, under date 

 of Feb. 20, 18 ( J5 7 concerning the last one of a series of pressure 



2H 2 



