jfiSSENTIAL AND PECUUAR. 



43 



ittotions of all the heavenly bodies depend upon the same power ; and the 

 principle thus struck out has of later years been still more extensively and 

 even more accurately applied to a solution of the most complicated phseno^ 

 mena. This principle in astronomy is denominated the centripetal force, 

 and the term is sufficiently precise for^all common purposes ; since, although, 

 speaking with perfect strictness, the central point of no solid substance is 

 the actual spot in which its attractive power is chiefly lodged, yet it has 

 been abundantly proved by Sir Isaac, that all the matter of a spherical 

 body, or a spherical surface, may, in generally estimating its attractive force 

 on other matter, be considered as collected in the centre of such sphere. 

 And hence, as all the celestial bodies are nearly spherical, their action on 

 bodies at a distance may be held the same as if the whole of the matter of 

 which they consist were condensed into their respective centres. 



To what extent in the heavens the power of gravitation ranges it is im- 

 possible to determine ; there can be little doubt, however, that it extends 

 from one fixed star to another, although its effects are too inconsiderable 

 to be calculated by man. It may possibly influence the progressive motion 

 of several of the stars, and, as I had occasion to observe in a preceding 

 lecture, is the cause to which Dr. Herschel ascribes the origin of the mate- 

 rial universe, which he supposed at one time, though he seems afterwards 

 to have modified his opinion, as we shall notice in our next study, to have 

 issued from an immense central mass of matter, pecuharly volcanic in its 

 structure, and to have been, consequently, thrown forth in different quanti- 

 ties, and at different times, by enormous explosions ; each distinct mass, 

 thus forcibly propelled, assuming, from the common law of projectiles, an 

 orbicular path, and endowed with the common property of the parent body, 

 ejecting in like manner minuter masses at different periods of time, which 

 have equally assumed the same orbicular motion, and ultimately become 

 planets to the body from which they have immediately issued, and which 

 constitutes their central sun. 



To produce such an effect, however, and in reality to produce any of 

 the motions which occur to us in the celestial bodies, the passivity of 

 matter is just as necessary as its gravitation. I have already observed 

 that, owing to its passivity, or vis inertije, matter has a tendency to per» 

 severe in any given state, whether of motion or of rest, till opposed by some 

 exterior power ; and that the path it assumes must necessarily be that of a 

 right line, unless the power it encounters shall bend it into a different direc* 

 tion. A projectile, therefore, as a planet, for example, thrown forth from 

 a volcano, would travel in a right line for ever, and with the exact velocity 

 with which it was thrown forth at first, if there were nothing to impede its 

 progress, or to alter the course at first given to it. But the attraction of 

 the volcanic sphere from which it has been launched does impede it, and 

 equally so from every point of its surface : the consequence of which must 

 necessarily be, that every step it advances over the parent orb it must be 

 equally drawn back or reined in, and hence its rectilinear path must be 

 converted into a curve or parabola, and a tendency be given to it to es- 

 cape in this line, which may be contemplated as a line of perpetual angles, 

 instead of in a direct course ; and as soon as the projectile or planet has 

 acquired the exact point in which the two antagonist powers precisely 

 balance each other — the power of flying off from the centre, communi- 

 cated to it by the volcanic impulsion, and which is denominated its centri- 

 fugal FORCE, and the power of falhng forward to the centre, communi- 

 cated by the attractive influence of the aggregate mass of matter which the 



V 



