ESSENTIAL AND PECULIAR. 



broad, though its bright or solid nucleus or planetary body was, not sup- , 

 posed to meetsure more than four hundred and twenty-eight miles. Its 

 perihelion path, or nearest approach to the sun, is stated at a distance of 

 ninety-seven millions of miles, its distance from the earth at ninety-three 

 millions. The comet of 1807 approached the earth within sixty-one 

 millions of miles, or about a third nearer the earth, and that of 1680 

 within a sixth of its diameter, or as near as 147,000 miles, its tail being of 

 a like length. 



There is one comet, however, that we seem to be somewhat better ac- 

 quainted with than with this that paid us so near a visit, or indeed than with 

 any other, from its having approached us visibly for four times in succession, 

 if not oflener. It was towards the beginning of last century that Mr, 

 Halley was struck with the remark, that the general elements and character 

 of the comets observed in 1531, 1607, and 1682, were nearly the same,; 

 whence he concluded that the whole formed but one identical body, that 

 took about seventy-six years to complete its eccentric orbit ; and hence, 

 although in consequence of this eccentricity, and its travelhng amidst a 

 range of heavenly bodies that are altogether invisible to us, and whose 

 influence seems to bid defiance to calculation, it is difficult to form an 

 estimate of its progress, he ventured to suggest, that it would appear again, 

 making due allowance for these incidents, towards the close of 1768, or 

 the commencement of 1759: and he had the high satisfaction of seeing 

 his prediction verified ; the comet passing its perihelion March 12th, 1759, 

 within the hmits of the errors of which he thought his results susceptible. 

 It is apparently this comet, which at this last period only excited the curi- 

 osity of astronomers and mathematicians, that in 1456, or four revolutions 

 earlier, towards the close of what are called the dark ages, spread such 

 consternation over all Europe, already, indeed, terrified by the rapid suc- 

 cesses of the Turkish arms, that Pope Calixtus was induced to compose 

 a prayer for the whole western church, in which both the Turks and the 

 comet were included in one sweeping anathema. 



Admitting the truth of Dr. Herschel's hypothesis, as we are now con- 

 templating it, it is possible that some of the lately discovered planets, which 

 are now attendant upon the sun, were formerly comets, whose orbits have 

 for ages been growing progressively more regular, as well as their consti- 

 tutional rudiments more dense ; and such, indeed, is the opinion of M. 

 Voight, and of various other philosophers on the continent. 



The object of the present and the preceding lecture has been to submit 

 a sketch of the most obvious properties belonging to matter, so as to 

 enable you to obtain a bird's-eye-view of the general phaenomena it is 

 capable of assuming, and the general changes it is necessarily sustaining. 

 From the quahties I have placed before you, of passivity, cohesibility, 

 divisibility, and attractions of various kinds, must necessarily result, ac- 

 cording to the intensity with which they are called into action, the phaeno- 

 mena of liquidity, viscidity, toughness, elasticity, symmetry of arrangement, n 

 soHdity, strength, and resilience. But the powers which thus perpetually 

 build up the inorganic world, and to this our survey has been entirely con- 

 fined, perpetually also destroy it : for the whole, as I have had occasion to 

 observe, is a continued circle of action ; a circle most wise, most harmo- 

 nious, most benevolent : and hence, as one compound substance decays, 

 another springs up in its place, and can only spring up in consequence of 

 such decay. 



There is, however, another lesson, if I mistake not, which we mav 



