2 6o THE POPULAR SCIENCE MONTHLY. 



what I think is a masterpiece of glass-working the pump which en- 

 ables me so readily to produce a vacuum unattainable by ordinary 

 means. 



The pump here at work is a modification of the Sprengel pump, 

 but it contains two or three valuable improvements. I cannot at- 

 tempt to describe the whole of the arrangements, but I will rapidly 

 run over them as illuminated by the electric light. It has a triple- 

 fall tube in which the mercury is carried down, thus exhausting with 

 threefold rapidity; it has Dr. McLeod's beautiful arrangement for 

 measuring the residual gas ; it has gauges 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 contrivance 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 complicated, is exhausting an apparatus 

 which has enabled me to pass from the mere exhibition of the phe- 

 nomena 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 ascertain 



1. What were the actual rays invisible heat, luminous, or ultra- 

 violet which caused this action ? 



2. What influence had the color of the surface on the action ? 



3. Was the amount of action in direct proportion to the amount 

 of radiation ? 



4. What was the amount of force exerted by radiation ? 



I required an apparatus which would be easily moved by the im- 

 pact 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 pendu- 

 lum represented by this horizontal line has a weight at the end. It is 

 supported on two fibres of glass, one stretched upward and the other 

 stretched downward, both firmly fastened at the ends, and also at- 

 tached to the horizontal 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 ap- 

 proaches the horizontal line the slower its rate of oscillation. If I 

 relax the tension, by throwing the horizontal beam downward, I get a 



