HOGG. — FRICTION AND FORCE DUE TO TRANSPIRATION. 129 



by measuring F, 2^ can be calculated for any other degree of exhaustion 

 and the result compared with the value of p obtained by the McLeod 

 gauge. If (IV) is the proper relation between F and x>, the values of 

 the latter obtained from (IV) and from the gauge should be the same 

 over any range of pressures where the results obtained by the gauge 

 are reliable. 



In Sutherland's hands, this formula has stood the test fairly well, but 

 it must be added that the experimental conditions from which the 

 data used by him were obtained did not well conform to the mathe- 

 matical conditions assumed in the deduction of the above formula. 

 Still, the results obtained by him indicate the direction in which to 

 look for the full solution of the problem. 



Transpiration Apparatus — Description. — Figure 3 is a horizontal 

 section of the instrument through the centre of the suspended vane. 

 The essential parts of it are a cylindrical glass vessel, A, Figure 3, 

 about 7.5 cm. in diameter, in which is fixed an annulus of mica 1.25 

 cm. wide, the plane of the annulus being a cross section of the vessel. 

 The vane, suspended in a manner to be described later, will pass 

 through the opening of the annulus, leaving about 0.75 mm. clear- 

 ance. The vane is 4.7 cm. in diameter, and is less than 0.1 mm. in 

 thickness. The vane is clamped between a shoulder and nut at B, 

 Figure 3. The supporting wire, C, is in turn fastened to another, 

 D, by collar and check-nut, U and V. D bears a mirror, E, and F 

 and G are counterpoises. At K is a short metal tube- shown in 

 vertical section at H, Figure ^a. The wire, I, Figure 3a, is furnished 

 with shoulder and check-nut, seen at L and J, and just fits the tube, 

 H. The fastening of the fibre is the same here as in the viscosity 

 apparatus. 



In Figure 4 is shown the arrangement \>y means of which the sus- 

 pended system can be raised or lowered without orienting, and 

 oriented without raising or lowering. A is a soft iron armature, D a 

 supporting swivel-head, the rod of which presses through guide-pieces, 

 E and F, and terminates in a second supporting swivel-head, G. To 

 the vertical rod is fixed a cross-bar, II, furnished with vertical guide- 

 pieces, I I, which pass through the wire loops, J J. To the armature, 

 A, the fibre clamp is fixed. The clamp is kept in the centre of the 

 tube by means of the wire, K, which has a loop through which the 

 clamp wire, L, just passes. M is another soft iron armature supported, 

 as sho^\Tl, on a screw passing through a nut, N, which, in order to 

 avoid complications from inequalities of expansion, is made of the same 

 material as the screw. 



VOL. XLII. — 9 



