462 
PROFESSORS A. W. REINOLD AND A. W. RUCKER ON 
The vertical cross wire of the telescope of the cathetometer being directed to the 
centre of the illuminated band, a plumb line is passed along the edge of the graduated 
circle, until it arrives at the point C, when it coincides with the cross wire. This 
point is noted on the circle. Similarly when the plumb line is at A, its image formed 
by reflection coincides with the cross wire. We thus know the arc AC and 
consequently the angles a and 0. 
Successive readings made in this wa,y rarely differed by more than 10', and as the 
mean of several determinations was always taken, the accuracy of the method was 
amply sufficient for our purpose. The values of a during the experiments described 
in this paper were 42° 41' and 2° 44'. 
The Hygrometer .—Fig. 4 represents the hair hygrometer and the thermometer 
attached to it. The hair was cleaned according to the method prescribed by 
Regnault. The index is of ivory, and is graduated in degrees of arc. When the 
instrument was placed under a receiver, dried by sulphuric acid, and left for 24 hours, 
the index stood at 0°. When the receiver was saturated with aqueous vapour 
the index stood at about 60°. In the experiments described, the hygrometer was only 
used for the purpose of ascertaining whether any change in the hygrometric state of 
the air surrounding the film took place while the film lasted. No attempt was made 
to obtain the actual values of the hygrometric state corresponding to the indications 
of the instrument. 
The position of the hygrometer in the case is shown at Y, fig. 5. A strip of glass 
was placed in front of it to protect it from the spray caused by the bursting of the 
films. 
Method of maintaining a constant hygrometric state .—In many of the experiments 
it was necessary to prevent as far as possible any absorption of vapour by or 
evaporation from the films. To check evaporation, the lower cup B was filled with 
liquid to within about 2 millims. of the edge. Two dishes containing the liquid and 
exposing an evaporating surface of about 20 square centims. were also placed in the 
inner chamber. On and after September 24 these were replaced by a much larger 
dish with an evaporating surface of about 52 square centims. Sheets of blotting- 
paper moistened with the liquid were attached to the back and one of the sides of the 
inner case. One of these dipped in a small vessel containing the liquid and could be 
moistened by a pipette introduced through a small hole cut in the back of the 
apparatus. 
As the inner glass case, though fitting the ebonite base board, I, sufficiently well to 
prevent the ready passage of air or aqueous vapour, was not absolutely air tight, 
small dishes containing the liquid with pieces of blotting paper, also moistened with 
the liquid, dipping into them, were placed in the outer case. A dish of the liquid 
was also placed in the chamber below the case. 
The air for forming the films was contained in a bladder, fitted with pressure 
boards. On being forced from the bladder it passed first through two tubes containing 
