PHYSICAL ASTRONOMY. 131 
influence on their motions. The comets, however, which are known to be 
bodies of very slight density, may experience some retardation, even if it be 
very slight, from the rare ether existing in space. The continual abbrevia- 
tion of the period of revolution of Encke’s comet, already several times 
returned, observed, and calculated, indicates conclusively enough that space 
is filled by a medium, and consequently is not absolutely empty. This 
abbreviation of the period of the above mentioned comet, is evidently a 
consequence of the resistance which the ether opposes to the course of the 
comet. The comet itself may by this means experience, one day or other, 
a very destructive catastrophe. Since it meets a resistance in its course 
around the sun, it naturally will not be able to retain its original orbit, but 
must by degrees move in arcs which lie nearer to the sun than those 
previously described in similar times. The orbit will, therefore, remain no 
longer an ellipse or a closed curve, but must become a spiral, terminating 
in the sun itself, since the comet will be more and more affected by his 
attraction, and consequently approach nearer and nearer ( pl. 6, fig. 26). 
The immediate consequence of this must be a gradually diminishing period 
of the comet, which will accomplish its course with greater and greater 
velocity, until finally it will be lost in the sun. 
The Sun’s Spots. 
51. Pl. 9, figs. 9-13 represent the black spots seen sometimes, with the 
assistance of the telescope, on the sun’s disk, of greater or less size, irregular 
shape, and surrounded by an ash grey border, generally of uniform breadth. 
The solar spots appear frequently to change not only their shape, as shown 
by figs. 10 and 11, but also their position on the sun’s disk. They are some- 
times very large, and their constant occurrence in connexion with the sola 
facule (fig. 9), as also their ash grey border, plainly indicate a common 
origin with these. Thus, for instance, their black spots may be seen to break 
out in the midst of these facule, or, inversely, facule arise in the places 
whence spots have just vanished. These facule are streaks, which, by 
their dazzling light, are distinguishable from the rest of the disk, and resem- 
ble, so to speak, veins of light. 
It is very rare that the spots on the sun are seen at a distance of more 
than 30° from his equator on each side. At their entrance on the sun’s 
disk they appear very small, and when they come near to the border of the 
sun, they are seen as black lines, becoming broader the nearer they are to 
the centre of the disk; they moreover seem to move in almost parallel lines 
from east to west over the sun’s disk: their true motion, however, is from 
west to east, as they would appear to an eye at the centre of the globe of 
the sun. A spot generally occupies from 12 to 13 days in crossing the 
visible disk of the sun. It is then invisible for a period of 14-15 days, but 
at the expiration of this time it appears in the same place in about 27 or 28 
days after the first appearance, to commence its second revolution. The 
paths of the spots appear towards the 10th of June and 10th of December as 
131 
