456 Morley and Brush — Gauge for the 



tube. Within these tubes are fixed two platinum points, on 

 the same level. The points must be rather accurately shaped. 

 To this U-shaped gauge is connected the tube c, and a thick- 

 walled rubber tube d, which can be compressed by the screw 

 e e ; d is continued by a glass tube used to adjust the level of 

 the mercury in a b when the apparatus is set up, and then 

 closed by fusion. By means of the screw and the rubber tube, 

 the level of the mercury in a b can be adjusted with any 

 needed degree of accuracy. 



This gauge is firmly cemented into a metal girder or beam. 

 This beam rests on two points at m, and on the point of a 

 micrometer screw at n. 



The instrument has to be mounted on a stable support. 

 Ours is placed on a brick pier having a recess for the tube c. 

 On the tops of the two projecting parts are cemented two 

 glass plates ; on one of them rest the two points rn and on the 

 other the point of the micrometer screw h. Guard rings keep 

 the points in the assigned position. 



The method of using the apparatus is simple. We must first 

 determine the reading of the micrometer when the pressure 

 in a and b has been made the same by putting them in free 

 communication by the mercurial valve shown. We turn k 

 till the depressions made by the two points in a and b seem 

 equal. Then we lessen the compression of d and so lower the 

 level of the mercury in a b till one of the depressions is barely 

 perceptible. These two steps are repeated ; we equalize and 

 then reduce the depressions till they are equal when made the 

 minimum msibile. The reading of h is then the reading for 

 equality of pressure in a and b. 



The method of comparing the depressions requires no 

 optical assistance. The observer places himself so as to see 

 the image of a horizontal window-bar reflected by the two 

 mercury surfaces. Moving his eye so as to cause the image to 

 pass across the depressions, their dimensions are easily com- 

 pared. As they are made smaller by lowering the level of the 

 mercury, the error of comparison becomes smaller. When 

 they are barely visible, this error may easily be made less than 

 0-00002 cm. ; after some practice, of course. 



When we wish to measure the difference of pressures in a 

 and b, we make a new reading in the same way. The 

 difference between the readings, taken with the length m n y 

 and the value of a turn of the screw, will define the inclination 

 which has been given to the apparatus in order to make the 

 two points again tangent to the two surfaces of mercury. 

 The linear distance between the points is determined when 

 the apparatus is constructed ; in our instrument, this was done 

 by measuring with a cathetometer while the tubes a b were 



