90 B.V. Marsh on the Distinguishing Features of Comets. 
of putting on the cometary condition, but that in the planets 
circumstances have not favored its development. Now, except 
ing the peculiar cometary appearances, the only feature in which 
it is known that all comets agree with one another and differ 
from the planets is the excentric form of orbit—the distance 
of a planet from the sun is almost constant, that of a comet variable 
—and it is to the variation in the comet’s distance from the sun, 
and the resulting contrast of condition whilst in different parts of 
the orbit that I would attribute the development of the comet 
ary phenomena. 
A planet revolves about the sun at a nearly uniform distance 
and the action of the sun’s rays upon it is nearly constant 
light and heat received from the sun diminishes as the square of 
the distance increases, the comet must alternately be subjected to 
conditions of the most extreme contrast. For example, the 
distance of Halley’s comet when in perihelion is 56 millions of — 
miles—in aphelion 3370 millions; so that in the former position 
it receives in a given time 3600 times as much heat and light as 
is passed in 15°7 hours—so that during a long series of years this 
comet remains exposed to the cold of these distant regions (the tem- 
heat than it receives from the sun. Sot only when the 
time for its perihelion passage approaches quite nearly that the 
lowest temperature of the mass is attai n rushes to 
e attainment of a different condition in reference to the 
other forces of which the sun seems to be the source. Being 
precipitated into the immediate vicinity of the sun in thisex 
mely negative condition the solar action upon it must pro 
hanges of the most violent kind, and since all chang® 
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