1876.] 
Mechanical Action of Light, 
249 
telegraphic wires to a central observatory, an exadt account 
can be kept of the proportion of sunlight received in different 
latitudes, and at various heights above the sea-level. Fur- 
thermore, our records of the comparative temperature of 
different places have been hitherto deficient. The tempera- 
ture of a country depends partly on the amount of rays which 
it receives diredt from the sun, and partly on the atmospheric 
and oceanic currents, 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 be- 
ginning of this ledture — “ What is the amount of force 
exerted by radiation ? ” Well, I can calculate out 
the force in a certain way, from data supplied 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 adted on, it is not 
difficult to calculate the amount of force required to defledt 
the beam through a given angle ; but I want to get a more 
diredt 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 before showing the experiment I want to 
illustrate the principle upon which it depends. Here is a very 
fine glass fibre suspended from a 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 3 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 750 grains, but gives way when additional weight is 
added. You see then the great strength of a fibre of glass, 
so fine as to be invisible to all who are not close to it, to 
resist a tensile strain. 
Now I will illustrate another equally important property 
of a glass thread, viz., its power to resist torsion. Here is a 
still finer glass thread, stretched horizontally between two 
supports ; and in order to show its position I have put little 
jockeys of paper on it. One end is cemented firmly to a 
wooden block, and the other end is attached to a little in- 
strument called a counter— a little machine for registering 
