274 THE POPULAR SCIENCE MONTHLY. 



instrument, and proving that in this case also the radiometer obeys 

 the law of inverse squares. 



This instrument, the principle of which I have illustrated to-night, 

 is not a mere toy or scientific curiosity, but it is capable of giving 

 much useful information in climatology. You are well aware that the 

 temperature, the rainfall, the atmospheric pressure, the direction and 

 force of the wind, are now carefully studied in most countries, in 

 order to elucidate their sanitary condition, their animal and vegetable 

 productions, and their agricultural capabilities. But one most im- 

 portant element, the amount of light received at any given place, has 

 been hitherto but very crudely and approximately estimated, or rather 

 guessed at. Yet it cannot be denied that sunlight has its effect upon 

 life and health, vegetable, animal, and human, and that its relative 

 amount at any place is hence a point of no small moment. The diffi- 

 culty is now overcome by such an instrument as this. The radiom- 

 eter may be permanently placed on some tall building, or high moun- 

 tain, and, by connecting it by telegraphic wires to a central observa- 

 tory, an exact account can be kept of the proportion of sunlight 

 received in different latitudes, and at various heights above the sea- 

 level. Furthermore, our records of the comparative temperature of 

 different places have been hitherto deficient. The temperature of a 

 country depends partly on the amount of rays which it receives direct 

 from the sun, and partly on the atmospheric and oceanic ctirrents, 

 warm or cold, which sweep over or near it. The thermometer does not 

 discriminate between these influences ; but the radiometer will enable 

 us now to distinguish how much of the annual temperature of a place 

 is due to the direct influence of the sun alone, and how much to the 

 other factors above referred to. 



I now come to the last question which I stated at the beginning 

 of this lecture, " What is the amount of force exerted by radiation ? " 

 Well, I can calculate out the force in a certain way, from data sup- 

 plied by this torsion apparatus (Fig. 4). Knowing the weight of the 

 beam, the power of the torsion fibre of glass, its time of oscillation, 

 and the size of the surface acted on, it is not difficult to calculate the 

 amount of force required to deflect the beam through a given angle ; 

 but I want to get a more direct measure of the force. I throw a ray 

 of light upon one of these instruments, and it gives a push ; surely it 

 is possible to measure the amount of this push in parts of a grain. 

 This I have succeeded in doing in the instrument behind me; but be- 

 fore showing the experiment I want to illustrate the principle upon 

 which it depends. Here is a very fine glass fibre suspended from an 

 horizontal bar, and I wish to show you the strength of it. The fibre 

 is only a few thousandths of an inch thick ; it is about three feet long, 

 and at the lower end is hanging a scale-pan, weighing 100 grains. 

 So I start with a pull of 100 grains on it. I now add little lead 

 weights, 50 grains each, till it breaks. It bears a pull of V50 grains, 



