CELI.STIAL PHENOMENA. 87 



Here, too> we find differences existing in the solidity or density 

 of the spheroid ally agglomerated matter. Our own solar sys- 

 tem presents all stages of mean density (or of the relation of 

 volume to mass.) On comparing the planets from Mercury 

 to Mars with the Sun and with Jupiter, and these two last 

 named with the yet inferior density of Saturn, we arrive, by 

 a descending scale — to draw our illustration from terrestrial 

 substances — at the respective densities of antimony, honey, 

 water, and pine wood. In comets, which actually constitute 

 the most considerable portion of our solar system with respect 

 to the number of individual forms, the concentrated part, 

 usually termed the head, or nucleus, transmits sidereal light 

 unimpaired. The mass of a comet probably in no case equals 

 the five thousandth part of that of the earth, so dissimilar are 

 the formative processes manifested in the original and perhaps 

 still progressive agglomerations of matter. In proceeding from 

 general to special considerations, it was particularly desirable 

 to draw attention to this diversity, not merely as a possible, 

 but as an actually proved fact. 



The purely speculative conclusions arrived at by Wright, 

 Kant, and Lambert, concerning the general structural ar- 

 rangement of the universe, and of the distribution of matter 

 in space, have been confirmed by Sir William Herschel, on 

 the more certain path of observation and measurement. That 

 great and enthusiastic, although cautious observer, was the 

 first to sound the depths of heaven in order to determine the 

 limits and form of the starry stratum which we inhabit, and 

 he, too, was the first who ventured to throw the light of inves- 

 tigation upon the relations existing between the position and 

 distance of remote nebulce and our own portion of the sidereal 

 universe. William Herschel, as is well expressed in the ele- 

 gant inscription on his monument at Upton, broke through the 

 inclosures of heaven {ccdorum perrupit claustra), and, like 

 another Columbus, penetrated into an unknown ocean, from 

 which he beheld coasts and groups of islands, whose true po- 

 sition it remains for future ages to determine. 



Considerations regarding the difTerent intensity of light in 

 stars, and their relative number, that is to say, their numeric- 

 al frequency on telescopic fields of equal magnitude, have led 

 to the assumption of unequal distances and distribution in spaca 

 in the strata which they compose. Such assumptions, in as 

 far as they may lead us to draw the limits of the individual 

 portions of the universe can not offer the same degree of math- 

 ematical certainty as that which may be attained in all that 



