REVIEWS—DONATI’S COMET. 73 
the effect of heat as it nears the sun, vastly expanding its dimensions ; 
there will be condensations and rarefactions in abundance, and the 
stratification will become complicated enough to meet most contingen- 
cies, but the median line will deviate from the radius vector in the 
backward direction. Now add a luminous action of some sort excited 
by the sun, (say “electricity,” remembering certain resemblances in the 
comet’s behaviour to that of the Aurora,) and instantaneously exerted 
in the directions of least resistance. Perhaps by this time we may 
have come to something not unlike Herschel’s “negative shadow,” im- 
pressed, however, not on the “luminiferous ether” but on the atmos- 
phere of the comet itself, and if we take into account the possibility of 
the existence of several centres of condensation instead of only one, 
the subsidary phenomena may not be impossible to explain. It seems. 
certain that the body of a comet is not confined merely to that part of 
it which is visible to us,—the discovery of the new “veil” may assure 
us of this,—and the diminution of volume of the head as it approaches, 
with the subsequent increase as it recedes from the sun suggests (as 
Newton remarked) an evaporation or transformation into non-illumi- 
nated gas of the nebulous substance, which is again condensed into 
the head. 
What is the ‘final cause’’ of comets, or what useful end do they 
serve in the plan of creation? Not to mention the moral effects 
they have exerted in past ages on the ignorance of mankind, nor 
the forgotten theory of Whiston, that a comet was the instrument of 
God’s wrath in the Noachian deluge, by so near an approach that the 
impulse of the resulting tide in the inner sphere of water was great 
enough to fracture the solid envelope of the earth, nor the strange 
conjecture of Buffon that the planets were bits of the sun chipped off 
by the dash of comets against him, nor that of Olbers that the asteroids 
were the fragments of a large planet broken up by collision with a 
comet, nor other groundless fancies of the same kind, we can assert 
that one good service has been rendered by them to philosophy, by 
enabling us to ascertain from their perturbations the masses of the 
planets, and perhaps also by showing that the nebulous matter of 
extra-planetary space is like common matter in its subjection to the 
law of gravitation. Newton, however, with a fertility of imagination 
which recalls to us Dr. Johnson’s saying :—‘‘I am persuaded that 
had Sir Isaac Newton applied to poetry, he would have made a 
very fine epic poem,”’—has suggested that the light and heat of 
the sun may be renewed and sustained by the comets which, moving 
in ever-contracting orbits, would ultimately fall in and be absorbed 
