their Appearance, Nature, and Origin. 7 
to modern theories of optics, we have to suppose that light exerts 
a certain pressure on the opaque bodies it encounters. 
The fact that we can get no experimental evidence of such a 
pressure is no argument against its existence, for its calcu- 
lated value is far too small to produce any effect which our in- 
struments could detect in the case of the familiar objects which 
surround us. 
But when we have to deal with the excessively fine dust, of 
which we suppose the material of the comet to be formed, the 
pressure of light may be large enough to overcome the sun’s 
gravitation. 
For a pressure is proportional to the area over which it acts, 
but the force of gravitation is proportional to the mass of the 
body, and therefore to its volume. 
Now if we have a cube of one inch side, made of material 
weighing one pound to the cubic inch, if we hold it up in the 
hand, the pressure upwards, which we have to exert upon its base 
to keep it balanced, must be exactly one pound weight; and 
this pressure is exerted upon an area of one square inch, so that 
it is one pound weight per square inch. 
But if we make a cube of side half an inch, of the same 
material, its volume is only one-eighth of what it was before, and 
its weight accordingly one-eighth of a pound. To hold it up, 
my hand has to exert upon its base, which is one-fourth of a 
Square inch in area, a total pressure of one-eighth of a pound 
weight, that is, of half a pound weight per square inch, or 
exactly half the previous pressure. 
And so on. If our cube, still made of the same material, be 
reduced to one-tenth of an inch side, its weight is reduced to 
one-thousandth of a pound, and the pressure I have to exert to 
balance gravity is one-thousandth of a pound weight upon an 
area of one-hundredth of a square inch—i.e. one-tenth of a 
pound weight per square inch. 
Thus as we diminish the dimensions of a body, keeping its 
density the same, a lesser and lesser pressure is found to balance 
gravitation. Ultimately it will be found that, if only our particles 
be taken small enough, any pressure intensity, however small, will 
be found able to overcome any gravitational attraction, however 
large. 
In a precisely similar way, if you take powdered lead and fair- 
sized lead shot, and let them fall, the shot will fall to the ground 
far more rapidly than the powdered lead. This is due to the 
resistance of the air, which is of the nature of a pressure, and 
is proportional to the area of the particles. In a vacuum the 
powdered lead and the shot would fall with the same rapidity. 
We see, then, that in the case of comets it is conceivable that 
the sunlight should exert a sufficient pressure on the minute 
