62 REVIEWS—DONATY'S COMET. 
only by the gravitating attraction of the sun, and the mutual attrac 
tion of its parts; that it should thus sweep round, unbroken (to use 
Herschel’s phrase) like a rigid rod, and this as in one case more than 
a hundred millions of miles long through a semicircle in less than two 
days, is absolutely impossible, being in defiance of the principle of 
gravitation and the common laws of motion. Before, however, enter- 
ing upon the discussion of any explanatory hypothesis, it will be well 
to examine somewhat minutely the actual phenomena manifested in 
the production of this strange appendage. 
Generally a comet is first seen as a telescopic objeet in the shape o 
a round nebulous body of a pale color, sometimes of equable illumina 
tion throughout, in other cases having its intensity increasing towards 
the centre, but without appearance of tail. As it approaches the sun, 
it begins to brighten and swell out slightly towards him, while on the 
opposite side the tail begins to run out in a luminous band of the 
breadth of the head. As it comes nearer and nearer the sun, much com- 
motion is seen to ensue in its head, indicated by rapid changes of bril- 
liancy and alternations of apparent density ; the tail now surrounding 
the head on the side towards the sun, and branching off in the oppo- 
site direction round the head into two streamers. These apparently 
are separated for some distance from the head, forming a bifurcation 
of the tail with a dark space interposed, but presently coalesce, widen- 
ing out as they recede, diminishing in brightness towards their ends, 
and less illuminated towards the median line between them. These 
streamers, however, are not straight but curved, the foremost streamer 
being convex towards the direction to which the comet is moving, 
and the hinder one being still more curved in the same direction, the 
former being also much the brighter of the two, and its outline more 
sharply defined and for a longer distance, so that in this state they 
are not inaptly compared by several of the observers to a quill pen or 
a bird’s wing. A better notion would be gained of this conformation 
by a glance at the beautiful figures in this volume than by any verbal 
description. The axis or median line of these streamers is at starting di- 
rected from the sun, but not exactly inthe line joining the sun and comet, 
generally being deviated backwards from the direction of the comet’s 
motion. In the comet of 1557, the deviation amounted to 20°, if the 
observation can be trusted. In this of 1858, the observations may be 
satisficd by a constant deviation of from 4° to 6°, the axis being sup- 
posed to be in the plane of the orbit, but Prof. Bond finds this latter 
