2 32 Mechanical Action of Light. [April, 
mercury is carried down, thus exhausting with threefold 
rapidity ; it has Dr. McLeod’s beautiful arrangement for 
measuring the residual gas ; it has guages in all directions, 
and a small radiometer attached to it to tell the amount of 
exhaustion that I get in any experiments ; it has a con- 
trivance for admitting oil of vitriol into the tubes without 
interfering with the progress of the exhaustion, and it is 
provided with a whole series of most ingenious vacuum- 
taps devised by Mr. Gimingham. The exhaustion produced 
in this pump is such that a current of electricity from an 
induCtion-coil will not pass across the vacuum. This pump 
is now exhausting a torsion-balance, which will be described 
presently. Another pump, of a similar kind but less com- 
plicated, is exhausting an apparatus which has enabled me 
to pass from the mere exhibition of the phenomena to the 
obtaining of quantitative measurements. 
A certain amount of force is exerted when a ray of light 
or heat falls on the suspended pith, and I wished to as- 
certain— 
First. What were the aCtualrays — invisible heat, luminous, 
or ultra violet — which caused this action ? 
Secondly. What influence had the colour of the surface 
on the aCtion ? 
Thirdly. Was the amount of aCtion in direCt proportion 
to the amount of radiation ? 
Fourthly. What was the amount of force exerted by 
radiation ? 
I required an apparatus which would be easily moved by 
the impaCt of light on it, but which would readily return to 
zero, so that measurements might be obtained of the force 
exerted when different amounts of light aCted on it. At 
first I made an apparatus on the principle of Zollner’s 
horizontal pendulum. For a reason that will be explained 
presently I am unable to show you the apparatus at work, 
but the principle of it is shown in the diagram (Fig. 3). The 
pendulum represented by this horizontal line has a weight 
at the end. It is supported on two fibres of glass, one 
stretched upwards and the other stretched downward, both 
firmly fastened at the ends, and also attached to the hori- 
zontal rod (as shown in the figure) at points near together, 
but not quite opposite to one another. 
It is evident that if there is a certain amount of pull upon 
each of these fibres, and that the pull can be so adjusted as 
to counteract the weight at the end and keep it horizontal, 
the nearer the beam approaches the horizontal line the slower 
