92 Mr. W. Sutherland on two New 



placing the axis of cylinder and piston horizontal and in one 

 of the lines where the horizontal plane through the axis cuts 

 the cylinder, piercing it with a small hole at its middle point ; 

 this hole establishes communication with a small chamber, 

 and through it passes an arm for supporting the piston in its 

 right position, the chamber being large enough to hold a 

 counterpoise to the piston attached to the other end of the 

 arm ; from the top of the chamber rises the fine vertical tube 

 which is to contain the quartz torsion fibre rigidly attached 

 to the arm and connected at the top of the tube with a torsion 

 head, whereby a sufficient couple can be applied to balance 

 the twisting effect of the pressure on the piston. In order 

 not to interfere with the vacuum in the apparatus the torsion 

 head should consist of a small magnet so pivoted that it can 

 be turned through any angle by a large magnet outside, and 

 so twist the quartz fibre through an angle measured by the 

 usual mirror arrangement. To bring the piston always to 

 the same position a mark on the counterpoise as seen through 

 a window in the little chamber is brought into alignment 

 with a mark on the window and a fixed point of reference 

 such as cross fibres at a convenient position outside. The 

 angle of torsion can be written equal to c / /(A // j9 + B /// + 1/p), 

 and three or more measurements of its value at pressures 

 determined by a M'Leod gauge of assured rehability furnish 

 the values of the constants or rather parameters d \ A", B 7// 

 for the instrument and the gas, and then any other pressure 

 can be calculated from an observation of the angle of 

 torsion. 



As to the material for the piston, Orookes's experience 

 seems to show that a disk of mica blackened on one side 

 would be eminently suitable ; a circular disk, out of which a 

 smaller one was cut, leaving an empty ring between the two, 

 would form both piston and cylinder, and there would only 

 remain to attach the solid ring inside the drum or vessel of 

 suitable shape which is to hold the gas under study; for 

 example, the solid ring could be cut of such a size that it 

 just fits into a shallow cylinder to which it can be cemented, 

 so that when the top and bottom of the cylinder are closed 

 by thin glass or metal plates, and when the piston is in 

 position, there are two shallow cylindrical chambers commu- 

 nicatino- through the annular space between piston and 

 cylinder. The junctions would have to be such that the 

 whole could be raised to 200° or 300° C. for thorough drying 

 and evacuation. The front of both the disk and ring of mica 

 being black and the back clear, there would be established a 

 fall of temperature through the annular interspace such as is 



