128 
HEAT AND NOT LIGHT A MOTIVE POWER ; OR, 
EXPERIMENTS WITH RADIOMETERS. 
By H. A. CUNNINGTON. 
rilHESE ingenious and beautiful little instruments were 
X invented by Mr. Crookes. I believe he at first supposed 
that they showed “ repulsion by radiation ” of heat ; since then 
he speaks of them as showing the “ mechanical action of light,” 
and has gone so far with his experiments as to weigh a ray of 
light, proving that the force of the light our earth receives 
from the sun is equal to fifty-seven tons per square mile. I 
have been experimenting for nearly three months with some of 
the radiometers made by Dr. Greissler, and obtained through 
Mr. Browning, who has kindly placed at my disposal some of 
the best he has had. As these little instruments are not gene- 
rally known, I will try and give a description of them before 
beginning an account of my experiments. Four little arms 
of equal length are fastened at right angles to one another, and 
attached to a socket of hard glass, in which the point of a 
needle works as a pivot. On the free end of each arm is 
fastened a vane, black on one side and white on the other, each 
one of the same size, and so placed that they shall be like a 
continuation of the arms, and be at right angles to the plane of 
revolution. These arms, vanes, socket, and needle are enclosed 
in a glass globe, with a hollow stem, which serves as the means 
of keeping the globe upright by fixing it in a hole in a short 
wooden stand. The air is exhausted from the globe by means 
of a Sprengel air-pump until the nearest approach to a perfect 
vacuum has been obtained. All my experiments seem to point 
to the conclusion that “ heat ” is the motive power, and not 
64 light ” ; that is, if they can be considered as separate agencies. 
A theory of the action of the heat has been suggested to me by 
my brother, Mr. Cecil Cunnington, which seems to be able to 
account for all the phenomena I have as yet observed. I will 
not give it until after the account of my experiments, as it will 
then perhaps be more intelligible. 
