PHYSICAL SCIENCE. 



XXV 



tlie year 1687 ; and many years elapsed before 

 it became generally known and understood. 



The principle of gravity thus fully established 

 and its consequences developed, was not mention 

 ed by Newton for the first time. Some curious 

 references to it are found in the writings of the 

 ancients ; and Copernicus, Kepler, and Hooke 

 entertained opinions respecting it approaching 

 much nearer to accuracy. " I do not think,' 

 says Copernicus, " that gravity is any thing else 

 than an appetency of the parts of the earth given 

 by the providence of the Supreme Being, that 

 by uniting together they may assume the form of 

 a globe. It is probable that tins same affection 

 belongs to the sun, the moon, and the fixed stars, 

 \vhich are all of a round form. 1 ' 



Kepler, in his great work on the Motions of 

 Mars, treats of gravity as a force acting naturally 

 from planet to planet, and particularly from the 

 earth to the moon. " If the moon and the earth 

 were not retained by some animal or equivalent 

 force, each in its orbit, the earth would ascend to 

 the moon by a 54th part of the interval between 

 them, while the moon moved over the remaining 

 fifty-three parts ; that is, supposing them of the 

 same density." This passage displays a curious 

 mixture of ignorance and knowledge respecting 

 the planetary motions. 



Hooke made a nearer approach to truth than 

 any of his predecessors. In his attempt to prove 

 the motion of the earth in 1674, he lays it down 

 as a principle, that the heavenly bodies have an 

 attraction towards their own centres, which ex- 

 tends to other bodies within the sphere of their 

 activity. The force of gravity he considered as 

 greatest nearest the body, though he could not 

 determine the rate of variation. These were 

 considerable advances, though his opinions were 

 mixed with much error and much ignorance. 

 Yet he was disingenuous enough, when Newton 

 had determined the law according to which 

 gravitation varies, to lay claim himself to the 

 discovery. 



Of all the physical principles that have been 

 hitherto made known, there is none so fruitful in 

 consequences as that of gravitation. The same 

 sagacity that led to the discovery was necessary 

 to trace its consequences. 



The mutual gravitation of all bodies being 

 admitted, it was evident that, while the planets 

 were describing their orbits round the sun, they 

 must mutually attract one another ; and hence, 

 in their revolutions some irregularities, from the 

 description of equal areas in equal times, might 

 be expected. But hitherto such irregularities 

 had been observed only in the revolutions of the 

 moon. This led Newton to inquire what the 

 forces were, which, according to the laws just 

 discovered, could produce the irregularities in 



question. The moon must be acted on not only 

 by the earth, but also by the sun ; and it was at 

 once evident that the force which was sufficient 

 to bend the orbit of the earth into the form of an 

 ellipse, must have a sensible effect on the orbit 

 of the moon. He showed that it is not the whole 

 force which the sun exerts on the moon that dis- 

 turbs her motion round the earth, but only the 

 difference between that force and the force 

 which the sun exerts on the earth. To obtain 

 exact measures of the disturbing forces, he sup- 

 posed the entire force of the sun on the moon to 

 be resolved into two, of which one always passed 

 through the centre of the earth, and the othet 

 was always parallel to the line joining the sun 

 and earth, consequently to the direction of the 

 force of the sun on the earth. The former of 

 these forces being directed to the centre of the 

 earth, does not prevent the moon from describ- 

 ing round the earth equal areas in equal times. 

 But the effect of it on the whole is to diminish 

 the gravitation of the moon to the earth about 

 one 358th part, to increase her mean distance in 

 the same proportion, and her angular motion by 

 about a 179th. He proved, by a very subtle in- 

 vestigation, that these forces would not sensibly 

 change the elliptical orbit of the moon, but that 

 the orbit itself would be rendered movable ; its 

 longer axis having an angular and progressive 

 motion, by which it advanced over a certain arc 

 during each revolution of the moon. This af- 

 forded an explanation of the motion of the 

 apsides of the lunar orbit, which had been ob- 

 served to go forward at the rate of 3 4' nearly, 

 during the time of the moon's revolution, in 

 respect of the fixed stars. 



But the exact quantity of the motion of the 

 apsides did not correspond with the diminution 

 of the moon's gravity, as above assigned. There 

 ivas, therefore, a cloud overshadowing this part 

 of the lunar theory, which was not dissipated till 

 a greater advance in mathematical knowledge 

 jut it in the power of subsequent astronomers to 

 nvestigate the subject completely. 



The line of the lunar nodes had been observed 

 o retrograde at the rate of 3' 10" every day. 

 Newton showed that the second of the forces in- 

 o which the solar action was resolved, being ex- 

 irted not in the plane of the moon's orbit, but 

 n that of the ecliptic,inclined to the former, at 

 n angle somewhat greater than five degrees, its 

 effect must be to draw down the moon to the 

 dane of the ecliptic, sooner than it would other- 

 vise arrive at it ; in consequence of which, the 

 ntersection of the two planes would approach, 

 is it were, towards the moon, or move in a direc- 

 ion opposite to that of the moon's motion, or 

 >ecome retrograde. From the quantity of the 

 olar force, and the inclination of the moon's 

 o K 



