HALLEY’S COMET—CAMPBELL. 957 
is more than 4,000,000 miles per day. If such motions exist, the con- 
stituents of the tail on one night are not the constituents of the tail of 
the following nights. Photographs of many comets taken on certain 
nights seem to bear no resemblance to those taken on the preceding 
or following night. The tails of the earlier dates have been driven 
off into space, scattered into invisibility, and entirely new tails have 
taken their places. The forces acting outwardly from the sun and 
responsible for these expulsions were mysterious, and it is only within 
the last ten years that a fairly satisfactory theory has been established. 
Half a century ago the great physicist, Clerk-Maxwell, in developing 
the electro-magnetic theory of light, deduced mathematically that the 
so-called light and heat waves, in striking upon any object, exert a 
pressure upon that object, very much as ocean waves falling upon 
the cliffs press against the obstructing rocks. The pressure due to 
hight and heat waves, called radiation pressure, is extremely slight; 
so slight, in fact, that skilled experimenters were unable to detect its 
existence for many years. At last, about the year 1900, a Russian 
physicist, Lebedew, was able to observe this effect; and a few months 
later two American physicists, Nichols and Hull, were even more 
successful, for their accurate observations showed a satisfactory 
agreement with the demands of Maxwell’s theory. 
Almost immediately the famous Swedish scientist, Arrhenius, ex- 
pressed his belief that in this pressure of the sun’s heat and light 
waves we have the force which forms comets’ tails. All the materials 
of a comet are necessarily attracted by the sun, according to the law 
of gravitation. There can be no doubt that they are also acted upon 
by radiation pressure. The former seeks to draw all into the sun, the 
latter to drive them into outer space. These are opposing forces. On 
the more massive parts of a comet, comprising the nucleus, radiation 
pressure is ineffective; and the nucleus moves along in its prescribed 
curve with remarkable precision. Not so with the finely divided 
materials of the coma and tail. Gravity acts as a function of a par- 
ticle’s mass, whereas radiation pressure’s action is dependent upon 
the surface-area of a particle in relation to its mass. As particles 
become smaller and smaller a size will be reached such that these 
opposing forces will be precisely balanced. Particles larger than 
these will be drawn nearer to the sun. Particles smaller will recede 
from the sun. 
What seems to take place in a comet is something like this: Minute 
particles of solid matter or molecules of gas are expelled from the 
nucleus chiefly on the side toward the sun, probably under the influ- 
ence of the sun’s heat. Radiation pressure acts upon these particles 
to turn them directly away from the sun; and the cloud of particles 
thus projected forms the tail. As the repellant forces act continu- 
ously, the particles must travel continuously faster; and this is the 
