HOGG. — FRICTION AND FORCE DUE TO TRANSPIRATION. 137 



with wool, was it possible, even with the help of the constant tempera- 

 ture room, to remove serious disturbances arising from this cause. It 

 was also necessary to prevent, as much as possible, light from shining 

 on any part of the apparatus, as this would produce unequal heating. 

 The light from the telescope lamp passed through a water window, and 

 was allowed to shine only when observations were being made. 



The trouble arising from electrical charges can readily be distin- 

 guished from those just mentioned, for, with it, the vane tends to take 

 up a fixed position, and force is required to change that position in 

 either direction. The force on the vane arising from unequal heating 

 is unidirectional. Trouble from electrical charges showed itself most 

 distressingly after an attempt to wrap the bulb in tin foil for the pur- 

 pose of minimizing outside heat effects. The friction between the 

 metal and the glass produced a very pronounced charge. It was 

 noticed also that after filling the vessel with air in the manner already 

 described the suspension, or rather the whole apparatus, had become 

 so charged that its attraction for a pith-ball could easily be detected. 

 A rather weak radioactive substance was placed within the brass shield 

 as near as possible to the vane, and after several days the charge had 

 disappeared. During the time when the box was being packed with 

 the wool the brass shield was kept to earth, and it has remained so 

 connected. 



By attending carefully to all of these points it was found possible to 

 control the vane so that after the current was turned on in the lamp and 

 before the shutter was opened there was no great change in the zero 

 reading. 



Since the object is to establish a relation between pressure and the 

 difference of pressure maintained between the ends of a transpiration 

 space, and since any surface along which there is a variation of tempera- 

 ture must give rise to transpiration, it is clear that if the entire bulb 

 of the instrument is illuminated, there will be various causes contribut- 

 ing to the total transpiration pressure, because the different parts of the 

 surface of the bulb will not be ev[ually heated. Now Sutherland has 

 developed equation (IV) on the assumption that the only transpiration 

 space contributing to the effect is annular in form. This is the simplest 

 form of space to deal with analytically, and the attempt to establish 

 the desired relation is likely to prove more successful w4th such a space 

 than with an irregularly shaped surface unequally heated. In the form 

 of the apparatus finally adopted the bulb is of a somewhat different 

 form from that shown in Figure 3. The mica ring is placed against 

 the conical posterior part of the glass vessel, so that the whole of the 

 vane and ring, together with that narrow part of the glass in contact 



