New Forms of Stereometers. 309 



The volumes of the several portions of the tube (i, 2, 

 3, 4) were accurately determined by calibration with 

 mercury. They were as follow : — 



ist Instruraent : (i) i6'39 cbc. 



(2) 10 '9 7 reading on the graduated stem. 



2nd Instrument: (i) 1 5*10 cbc. 



(2) 1 1 -06 „ 



(3) 12-15 » 



(4) lo-oo „ 



In order to test the accuracy of these instruments, the 

 volume of a brass cylinder was determined by their aid, 

 and compared with that found by weighing in water. 



To carry out a dilatation experiment the cylinder is 

 placed at the uppermost portion of the tube, the sliding 

 tube adjusted so that the mercury stands exactly at the 

 intersection of the two arms of the etched cross, and the 

 well-greased glass plate then applied, care being taken that 

 the level of the mercury is not altered by this operation. 

 The height of the mercury in the movable tube is then read 

 off on the scale. 



The movable tube is then lowered, and adjusted so that 

 the mercury in the other branch is exactly at the level of 

 the second constriction, and the height of the mercury read 

 off as before (in the movable tube). The distance between 

 the two constrictions can be read off once for all, and is, of 

 course, constant. Finally, the height of the barometer is 

 taken. 



If 



Vi be the volume of the air in the top compartment 



of the tube, and 

 V2 be the volume of the tube down to the second 



constriction. 

 Pi the height of the barometer in mms. 

 P2 the pressure of the air after dilatation, and 

 X the volume of the cylinder introduced, 



