1868. | Sir John Herschel and Modern Astronomy. 193 
few sentences from one of his popular Essays; he is writing of 
Donati’s comet :—“It was not till the 14th August, or seventy- 
three days after its first discovery, that it began to throw out a 
tail, and to become a conspicuous object. Very soon after this, its 
first appearance, a slight but perceptible curvature was perceived in 
the tail, which on the 16th September had become unmistakable, 
and continued to increase in amount as the latter extended in appa- 
rent dimensions, till it assumed at length that superb aigrette-lke 
form, like a tall plume wafted by the breeze, which has never pro- 
bably formed so conspicuous a feature in any previous comet. To 
a certain extent, it 1s a common enough feature in the tails of 
comets, and is usually regarded as conveying the idea of their 
moving in a resisting medium, in a space, that is to say, not quite 
empty, as smoke is left behind a moving torch. But this is a very 
gross and inadequate conception of the peculiarity in question. The 
resistance of the ‘ether,’ such as the phenomena of Encke’s comet, 
already noticed, may be supposed to indicate, is far too infinitesi- 
mally small to be competent to produce any perceptible deviation 
from straightness. Nor is it at all necessary to resort to any 
such explanation of the fact. Such an appearance would naturally 
arise from a combination of the motion the matter of the tail had 
(in participation with that of the nucleus) with the impulse given 
it by the sun, each particle of it describing—from the moment of 
quitting the head, an orbit quite different from that of the latter, 
being under the influence of a repulsive force directed from the 
sun —a curve of the form, called by geometers an hyperbola, nearly 
approaching to a straight line, and having its convexity turned 
towards the sun.” 
From time to time we hear of evidences of this repulsive force, 
existing as a power belonging to masses of matter. This must not 
be confounded with the repellant power manifested by an electrically 
excited body, or by that seen in the repulsion of the similar poles 
of magnetic bodies. With humility we venture to inquire, is not 
this repulsive force of the sun as pure an hypothesis as the resisting 
ether of stellar space ? 
In the ‘Geological Transactions for 1832’ there appeared a 
paper by Sir John Herschel “ On Astronomical Causes which may 
influence Geological Phenomena.” 
The subject of this memoir is an example of much originality 
of thought. 
With regard to the operation of astronomical causes upon 
climate, especially as elucidating former varieties of climate in 
geological history, Sir John Herschel refers to the influences of 
the sun and moon. The moon’s mean distance is now on the 
decrease ; also, the eccentricity of the lunar orbit is subject to 
fluctuations ; both these causes would produce differences in the 
