MATTER FORMED OF SMALL' "PARTICLES 



by inclining it suitably. Any air now remaining may be swept 

 out, by closing the open end with the thumb, and causing the 

 air in the shorter branch to pass to the top of the longer 

 branch and down again. Mercury is now poured out 

 until it stands about half-way up the shorter branch, 

 and the tube is supported vertically. It will be 

 noticed that there is an empty space at the top A, 

 provided the tube be long enough to allow the vertical 

 distance between the two columns to be more than 

 760 m.m. If the tube be now attached to a graduated 

 scale this vertical distance may be exactly measured, 

 and observations will reveal variations from time to 

 time. 



The vertical distance between the two surfaces of 

 mercury may be most accurately measured by using 

 a cathetometer, which is an instrument consisting of 

 an accurately graduated and rigid bar firmly fixed on 

 a support carrying levelling screws, by which it may 

 be made perfectly vertical. Spirit levels fixed upon 

 a moveable portion will show when this condition 

 is reached. To the moveable portion is rigidly fixed 

 the reading telescope, which revolves in a horizontal 

 plane only, and a vernier which is so arranged that 

 the vertical movement of the telescope may be 

 accurately read. The cross- wires of the telescope are first of 

 all focussed upon some distant object, and then the telescope 

 replaced in position. The vertical distance between any two 

 points is then equal to the distance obtained from the readings 

 of the vernier, although the telescope may have been moved 

 horizontally through any angle in making the cross-wires 

 coincide with the required points. 



68. The Use of the Cistern Barometer. The cistern baro- 

 meter consists of a long straight tube, closed at one end, which 

 has been completely filled with pure mercury and then inverted 

 in a cistern holding mercury. The atmosphere, pressing upon 

 the surface of the mercury in the cistern, supports a column of 

 mercury within the tube, just as the atmospheric pressure on 

 the surface of the mercury in the shorter limb of the tube, 



