SECTION X.

DIMENSIONS OF NUCLEI AND TAILS.

Real dimensions of the nuclei and atmospheres of various comets-Uncertainty of these elements; variations of the nucleus of Donati's comet--Observations of Hevelius upon the variations of the comet of 1652-Do cometary nebulosities diminish in size when their distance from the sun decreases ?-Encke's comet considered in regard to this question at its apparitions in 1828 and 1838.

THE observations that we have just recorded give an idea of the brightness of cometary light, and the intensity to which that brightness may attain; but they afford no certain indication concerning the dimensions of cometary nuclei or atmospheres. Upon this point we are about to give the result of a few measurements; but these measurements, it must be understood, are not so exact as those of the bodies of the solar system, the planets, the moon, and sun. The uncertainty we speak of does not arise from the difficulties experienced in the determination of the measures themselves, although they contribute to it, cometary nuclei being often as deficient in a clear and well-defined outline as the nebulosities; but what more especially prevents us from regarding the numbers we now give as constant, and therefore characteristic elements of the comets to which they belong, is the continual variation to which the different parts of the head are subject during the time of the comet's apparition.

The following two tables contain the values obtained for the dimensions of various cometary nuclei and atmospheres, arranged in order of magnitude:

On comparing these tables we find that the six comets, 1799, I., 1811, I., 1807, 1847, I., 1780, I., and 1843, I, whose nuclei and atmospheres have been both measured, do not occupy the same relative positions in each. This is strikingly shown in the case of the great comet of 1811, whose some

what small nucleus was surrounded by an immense nebulosity, as is evident from the large number in the last line of the second table. The volume of the nucleus was only equal to the 6,300th part of the volume of the earth, whilst that of the coma was 2,800,000 times greater than the volume of the earth; that is to say, more than double the volume of the

sun.

In order to justify the remarks at the beginning of this section, let us take for example the beautiful comet of Donati, whose physical elements have been so carefully studied by Bond. The diameter of 5,600 miles, given in the table, has reference to the nucleus on July 19. On August 30 this diameter was reduced by one-sixth, and measured no more than 4,660 miles. It continued to decrease until October 5, on which day it did not exceed 400 miles, less than th of its diameter on July 19. The next day it attained 800 miles, having doubled its dimensions between one day and the next, the volume of the nucleus having thus been increased in the proportion of 1 to 8. Finally, on October 8 the diameter attained a new maximum of 1,120 miles, and on the 10th was reduced again by one-half, viz. to 630 miles. We do not now enter into the significance of these rapid variations, of which we shall have to speak hereafter, when treating of the physical constitution of cometary nuclei. It will then be seen that these variations appear to be connected with the changes of distance between the nucleus of the comet and the sun.

Hevelius, in the sixth book of his Cometographia, describes the physical aspect of the comet of 1652, the magnitude of the head and tail, together with the brilliancy and the colour of their light. He observes that, the apparent dimensions of the comet having diminished day by day, this diminution was the natural result of the continually increasing distance between

the comet and the earth, but that in reality the absolute size of the comet was increasing day by day. This observation, the value of which Pingré denies, because he does not believe that Hevelius could have measured with sufficient accuracy the dimensions of the comet or calculated its distances from the earth, has since been generalized, and several astronomers, including Newton, have remarked that the diameters of cometary nebulosities increase in proportion as the comet becomes more and more distant from the sun. Arago observes that the comets of 1618, II., and 1807, manifestly exhibited this phenomenon. It has, however, been better exemplified by Encke's comet in its two apparitions of 1828 and 1838. The following table shows these remarkable variations:

The diminution of the diameters is much more rapid than that of the distances from the sun; the six distances decrease, in fact, in the proportion of the numbers 100, 90, 65, 58, 50, and 36, whilst the corresponding diameters are to each other as the numbers 100, 81, 38, 25, 14, and 4; the distances being at length reduced to a third nearly, whilst the diameter is twenty-six times less; and if we pass from the diameter to the volume of the nebulosity, it will be found that between October 28 and December 24 the volume was reduced to the 17,600th part of its original value.

We now proceed to the variations exhibited by the same comet in 1838, the elements of which are as follows:

Fig. 38.-Encke's comet, according to the observations of Schwabe. 1. October 19, 1838; 2. November 5; 3. November 10; 4. November 12.

From October 9 to December 17 the distance of the comet from the sun was reduced in the proportion of four to one, while the real diameter of the nebulosity was reduced to the