This power of gravitation acts proportionably to the quantity of matter in bodies,a truth which Sir Isaac has demonstrated by experiments.This new discovery has been of use to show that the sun (the centre of the planetary system)attracts them all in a direct ratio of their quantity of matter combined with their nearness.From hence Sir Isaac,rising by degrees to discoveries which seemed not to be formed for the human mind,is bold enough to compute the quantity of matter contained in the sun and in every planet;and in this manner shows,from the simple laws of mechanics,that every celestial globe ought necessarily to be where it is placed.
His bare principle of the laws of gravitation accounts for all the apparent inequalities in the course of the celestial globes.The variations of the moon are a necessary consequence of those laws.
Moreover,the reason is evidently seen why the nodes of the moon perform their revolutions in nineteen years,and those of the earth in about twenty-six thousand.The several appearances observed in the tides are also a very simple effect of this attraction.The proximity of the moon,when at the full,and when it is new,and its distance in the quadratures or quarters,combined with the action of the sun,exhibit a sensible reason why the ocean swells and sinks.
After having shown by his sublime theory the course and inequalities of the planets,he subjects comets to the same law.The orbit of these fires (unknown for so great a series of years),which was the terror of mankind and the rock against which philosophy split,placed by Aristotle below the moon,and sent back by Descartes above the sphere of Saturn,is at last placed in its proper seat by Sir Isaac Newton.
He proves that comets are solid bodies which move in the sphere of the sun's activity,and that they describe an ellipsis so very eccentric,and so near to parabolas,that certain comets must take up above five hundred years in their revolution.
The learned Dr.Halley is of opinion that the comet seen in 1680is the same which appeared in Julius Caesar's time.This shows more than any other that comets are hard,opaque bodies;for it descended so near to the sun,as to come within a sixth part of the diameter of this planet from it,and consequently might have contracted a degree of heat two thousand times stronger than that of red-hot iron;and would have been soon dispersed in vapour,had it not been a firm,dense body.The guessing the course of comets began then to be very much in vogue.The celebrated Bernoulli concluded by his system that the famous comet of 1680would appear again the 17th of May,1719.Not a single astronomer in Europe went to bed that night.However,they needed not to have broke their rest,for the famous comet never appeared.There is at least more cunning,if not more certainty,in fixing its return to so remote a distance as five hundred and seventy-five years.As to Mr.Whiston,he affirmed very seriously that in the time of the Deluge a comet overflowed the terrestrial globe.And he was so unreasonable as to wonder that people laughed at him for making such an assertion.The ancients were almost in the same way of thinking with Mr.Whiston,and fancied that comets were always the forerunners of some great calamity which was to befall mankind.Sir Isaac Newton,on the contrary,suspected that they are very beneficent,and that vapours exhale from them merely to nourish and vivify the planets,which imbibe in their course the several particles the sun has detached from the comets,an opinion which,at least,is more probable than the former.But this is not all.If this power of gravitation or attraction acts on all the celestial globes,it acts undoubtedly on the several parts of these globes.For in case bodies attract one another in proportion to the quantity of matter contained in them,it can only be in proportion to the quantity of their parts;and if this power is found in the whole,it is undoubtedly in the half;in the quarters in the eighth part,and so on in infinitum.
This is attraction,the great spring by which all Nature is moved.
