GRAVITY. 



GRAVITY, a term used by physical 

 writers to denote the cause by which all 

 bodies move toward each other, unless 

 prevented by some other force or obsta- 

 cle. The most familiar effect, and that 

 which continually obtrudes itself on our 

 notice, is the weight of bodies, or their 

 tendency toward the centre of the earth. 

 It has not been ascertained, or rendered 

 probable, that gravity is a secondary pro- 

 perty of matter ; that is to say, that it 

 flows from any of the other known origi- 

 nal properties. Sir Isaac Newton, how- 

 ever, was of opinion, that our reasonings 

 on the subject might be simplified, by 

 supposing it to depend on a prodigiously 

 elastic and rare fluid, by him called ether, 

 and assumed to possess an increasing de- 

 gree of condensation, in parts of space 

 more and more remote from the various 

 masses of matter. According to this doc- 

 trine, a falling body moves, because it is 

 pressed toward the rarer parts of this ex- 

 tended fluid. We shall leave this theory 

 to its merits, as being neither very per- 

 spicuous, nor much related to our subject. 

 Bergman, and others, have considered 

 the chemical and cohesive attractions to 

 be one and the same with the attraction 

 of gravity, but modified in its laws, by 

 variations in the masses, densities, and 

 distances of the particles of bodies. Ma- 

 ny difficulties appear at first sight to offer 

 themselves against this supposition. But 

 in truth it cannot be examined at first 

 sight ; and requires to be submitted to 

 the rigour of mathematical investigation, 

 which has not yet been done. 



The phenomena of particular gravity, 

 or that which respects the earth, or by 

 which bodies descend or tend towards 

 the centre of the earth, are as follow : 



1. All circumterrestrial bodies do here- 

 by tend towards a point, which is either 

 accurately, or very nearly, the centre of 

 the magnitude of the terraqueous globe. 

 Not that it is meant that there is any vir- 

 tue or charm in the point called the cen- 

 tre, by which it attracts bodies ; but be- 

 cause this is the result of the gravitation 

 of bodies towards all parts of which the 

 earth consists. 



2. In all places equidistant from th6 

 centre of the earth, the force of gravity 

 is nearly equal. Indeed, all parts at the 

 earth's surface are not at equal distances 

 from the earth's centre, because the 

 equatorial parts are higher than the polar 

 parts by about seventeen miles ; as has 

 been proved by the necessity of making 

 the pendulum shorter in those places, be- 

 fore it will swing seconds. In the new 

 "Petersburg Transactions," vol. 6 and 



7. M. KraflTt gives a formula for the pro- 

 portion of gravity in different latitudes on 

 the earth s surface, which is this : 



y == (14-0.0052848 sine 1 A),g-; 



where g denotes the gravity at the equa- 

 tor, and y the gravity under the other la- 

 titude A. 



3. Gravity equally affects all bodies, 

 without regard either to their bulk, figure, 

 or matter : so that, abstracting from the 

 resistance of the medium the most com- 

 pact and the most loose, the greatest and 

 the smallest bodies would all descend 

 through an equal space in the same time, 

 as appears from the quick descent of 

 every light body in an exhausted receiv- 

 er. The space which bodies do actually 

 fall in vacuo, is 16J_ feet in the first se- 

 cond of time, in. the latitude of London ; 

 and for other times, eithei greater or less 

 than that, the spaces descended from rest 

 are directly proportional to the squares of 

 the times, while the falling body is not 

 far from the earth's surface. 



4. This power is the greatest at the 

 earth's surface, from whence it decreases 

 both upwards and downwards; but not 

 both ways in the same proportion ; for, 

 upwards, the force of gravity is less, or 

 decreases as the square of the distance 

 from the centre increases ; so that at a 

 double distance from the centre above 

 the surface, the force would be only one- 

 fourth of what it is at the surface ; but be- 

 low the surface, the power decreases in 

 such sort, that its intensity is in the direct 

 ratio of the distance from the centre ; so 

 that at the distance of half a semi-diameter 

 from the centre, the force would be but 

 half what it is at the surface ; at one-third 

 of a semi-diameter the force would be but 

 one-third, and so on. 



5. As all bodies gravitate towards the 

 earth, so does the earth gravitate towards 

 all bodies ; as well as all bodies towards 

 particular parts of the earth, as hills, &c. 

 which has been proved by the attraction 

 a hill has upon a plumb line, insensibly 

 drawing it aside. Hence the gravitating 

 force of entire bodies consists of those of 

 all their parts ; for, by adding or taking 

 away any part of the matter of a body, its 

 gravity is increased or decreased, in the 

 proportion of the quantity of such por- 

 tions to the whole mass. Hence, also, the 

 gravitating powers of bodies at the same 

 distance from the centre are proportional 

 to the quantities of matter in the bodies. 



General or universal gravity, is that by 

 which all the planets tend towards one 

 another ; and, indeed, by which all bodies 



