PEOE. 0. REYNOLDS ON THE FORCES CAUSED BY THE 
thermometers did not show signs of moving for some time. It also seemed probable 
that the immediate surface which was exposed to the light, besides coming to its tem- 
perature almost instantaneously, would probably assume a higher temperature than that 
which would be communicated through the material. In order to show this it 
occurred to me to construct 
A New Photometer. 
This instrument consists of two very thin hollow glass globes, 2\ inches in diameter, 
connected by a siphon-tube ■§• inch internal diameter. 
One of the globes was blackened on the inside with lampblack over one hemisphere, 
and the other was whitened with chalk in a similar manner ; the siphon-tube was filled 
with oil, the air within the globes was carefully dried, and they were sealed. The two 
clean sides of the globes are turned in the same direction, so that any light entering 
through these clean sides falls equally on the blackened and whitened surfaces within. 
The air within instantly commences to receive heat in proportion to the temperature 
of these surfaces, and, expanding, moves the liquid in the tube. 
By comparing the volume of ascertain length of the tube with the volume of the globes, 
the distance which the liquid moves for 1 degree difference of temperature has been 
found : 1 inch means 2-2 degrees. A scale having been fixed to the tube, the effect of 
light to cause a difference of temperature in the air can be read off. 
There is, however, still one difficulty : the air within the globes does not arrive at the 
temperature of the surfaces, as these do not entirely enclose it. All that can be said is 
that it is proportional, probably about \ or rather more. 
This difference may, however, be set off against the difference which must exist in the 
mean temperature of the vanes of the mill, and what it would be if they remained 
steadily perpendicular to the light. As it is, each part of the surface of the vane is only 
exposed to the light for half its time, and then at varying angles ; so that the light that 
it receives bears to the light which would fall on it, if fixed and perpendicular, the ratio 
of the diameter to the circumference of a circle, i. e. the ratio -. In the case of the 
7 T 
photometer the ratio of the section of the intercepted beam to the whole surface of the 
