HOaa. — FRICTION AND FORCE DUE TO TRANSPIRATION. 131 



By turning M alone, the whole suspended system is raised or lowered ; 

 while by turning A alone, a twist is given to the fibre while the height 

 of the system is unchanged. Magnets, similar to the one used for 

 turning the armature in the viscosity apparatus, and mounted so that 

 they are entirely free from the glass part of the apparatus, are used to 

 turn the armatures A and M. 



Openings were left at 0, P, Q, R, S, and at a point directly beneath 

 the suspending fibre, until theannulus and vane with its support- 

 ing wire, C, were put in place, the wire D, bearing mirror and counter- 

 poise, screwed into the collar U, and the adjusting apparatus bearing 

 the fibre and the wire I placed in position. When J had been tight- 

 ened, the whole suspension was raised by turning M. This operation 

 was carried out in order to determine w'hether the wire D had been 

 screwed just far enough into the collar U so that, when the system 

 was raised, the plane of the vane might be parallel to that of the 

 annulus. To remedy any want of parallelism, the suspended system 

 was lowered again, and the collar U turned with respect to D. After 

 going through the process of adjusting many times, the vane was finally 

 placed in the proper adjustment, which was maintained by means of 

 the nut, V. 



In order to avoid breaking the fibre, during the process of closing 

 the various openings in the apparatus, the suspension was lowered to 

 resting loops not shown in the figure. All of the openings mentioned 

 above were now closed save P, to which the connecting tubes shown in 

 Figure 5 were joined. 



The McLeod Gauges. — Two McLeod gauges of the form shown at 

 A, Figure 5, were used. They have different factors, one of them being 

 suited for the measurement of comparatively high pressures, while the 

 other, the highest factor of which is about 69,(H)(), is suited for the high- 

 est vacua. At B B are air-traps to prevent small bubbles of air, which 

 may have adhered to the glass tube below, from slowly rising and de- 

 stroying the vacuum. As it was necessary to keep the mercury in the 

 gauge for some time and to keep it clean, the ordinary method of rais- 

 ing the mercury by raising a reservoir of it attached by rubber tubing 

 to the barometer tube of the gauge, was replaced by the method which 

 Figure 5 will explain^ Here are two reservoirs C and D, attached to 

 their respective gauges. The entrance to each is fitted with a stop- 

 cock, and the single leading tube has a three-way cock at E. F is an 

 air-tight reservoir, S a pipe leading to the water tap, and T a waste 

 tap. Water rises through S and forces air from the top of F against 

 the mercury surfaces in C and D. 



