Letter from John Craig to unidentified recipient, dated 7 April 1727
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According to your desire I have here sent you a short account of the surprising discoverys & improvements, which the incomparable Sir Isaac Newton has made in the most curious & abstruse parts of Natural philosophy: you know Natural Philosophy had been turn'd into a Philosophick Romance by the famous Des Cartes, & the world was so delighted with it, that they took it for a true & real account of Nature till Newton publish'd his Mathematical Principles.
Astronomy certainly the noblest part of Philosophy, the System of the heavens gives a glorious & an amazing prospect; in them we see the Sun, the moon, the Planets, the Stars, & sometimes Cometes moving, & their motions have (in all ages) rais'd mens curiosity to inquire into the causes which produced them: This set men upon making (by various ways) Observations concerning their motions, & accordingly they have been made with indefatigable labour by many Astronomers especialy by the Noble Dane Tycho Brahe, & our own countrey man Mr Flamstead. But all of them being ignorant of the cause of the various motions & phænomena, which had been observ'd, therefore they had recourse to certain Hypotheses, by which these motions & appearances might be accounted for. But these being insufficient Astronomy remain'd still in the dark till the immortal Newton gave us his Philosophical Principles of Natural Philosophy. Now we know by Mathematical Demonstrations that the Sun is the center of our System, that the Planets perform their revolutions about the sun by the same principle of gravity toward it, which attracts a bullet shot out of a Gun (or any other projectile) towards the Earth. In like manner we know that for the same reason the Moon moves about the Earth, Iupiters four moons about him, & Saturns five moons about him. And hence we know that the irregularity observ'd in the Moons motions proceeds from its being attracted both by the Sun & Earth. And, by the same universal Principle of Attraction or Gravitation, Newton has given us a plaine & natural account of the Sea's ebbing & flowing. He has likewise shown us that Comets do perform their regulare revolutions about the Sun after the same manner as the Planets do theirs, but in vastly longer Orbits & Periods of time; for their Orbits are extended far higher than Saturn, & when they descend into our planetary System they approach nearer than the moon does to the Sun even so near as to be allmost set on fire by the Sun; for he found by computation that the Comete of the year 1680 in its nearest approach to the sun (which was upon Decemb: 8th) was 2000 times hotter than red hot iron. And hence he infers that the tailes of Cometes are a thin Vapour which their great heat emitts out of them. From this beautifull System of our Solar System thus demonstrated by Sir Isaac Newton it is very probable that the System of the whole universe consists of an infinite number of fix'd Stars or Suns whereof every one is the center of a Planetary System not unlike this which revolves about our Sun which is one of those Stars, & whose motions are subject to the like laws of Gravitation, as these which govern our planets & their respective Moons, & Comets
The next instance of his great penetration into Nature he has given in his Treatise of Light & Colours. Before this we had nothing but the precarious Hypotheses of Philosophers for explaining the Properties of Light & assigning the cause of Colours; so that we may say, Light it self was left in darknes till Newton arose & gave us a full discovery of it, & a compleet Theory founded upon a great number of curious Experiments frequently repeeted with the greatest accuracy & niceness possible. And now we know that Light is a combination of rays proceeding from the Luminous body; that these rays are not (as was formerly suppos'd) subject to the same degree of Refraction & reflection; & that the rays which are equaly refrangible do constitute one colour, & consequently that there are as many (& no more) different Original Colours as there are different parcells of rays, when each parcel is subject to the same degree of refraction, & every parcel has its own peculiar degree. He shows us how to separate a beam of Light into all its different parcells of rays & finds them to be only seven, & therefore that there are only seven original colours, viz. Red, Orange, Yellow, Green, Blue, Indigo, Violet; & that in every one of these there is an infinite number of degrees, exempli gratia in Red there is an infinite number between the first or deepest red & the last which terminates at the Orange colour. Then he shows how all the other colours are produced by mixing & compounding the rays of two or more of these seven Original Colours; So that we can tell what will be the colour resulting from the Mixture before that mixture is made. All these things are demonstrated by a vast variety of experiments which are no less delightfull to the Sight than the conclusions drawn from them are to the Mind. It would be endles to enumerate all the curious discoverys he has made in this Treatise, but I must not pass over that, whereby he shows how to know the bigness of the component particles of Natural bodys by their colours.
I shall not tell you what great improvements he made in Geometry & Algebra, by which he was enabled to finish the two foremention'd Books. But it is proper to acquaint you that his great application in his inquirys into Nature did not make him unmindfull of the great Author of Nature; they were little acquainted with him, who imagine that he was so intent upon his studys of Geometry & Philosophy as to neglect that of Religion & other things subservient to it. And this I know that he was much more fellicitous in his inquirys into Religion than into Natural Philosophy; & that the reason of his showing the errors of Cartes's Philosophy, was because he thought it was made on purpose to be the foundation of infidelity. And Sir Isaac Newton to make his inquiries into the Christian Religion the more successfull had read the ancient writers and Ecclesiastical Historians with great exactnes, & had drawn up in writing great Collections out of both; &, to shew how earnest he was in Religion, he had written a long explication of remarkable parts of the old & new Testament, while his Understanding was in its greatest perfection, least the Infidells might pretend that his applying himself to the study of Religion was the effect of Dotage. That he would not publish these writings in his own time because they show'd that his thoughts were some times different from those which are commonly receiv'd, which would ingage him in disputes, this was a thing which he avoided as much as possible. But now its hop'd that the worthy & ingenious Mr Conduit will take care that they be publish'd that the World may see that Sir Isaac Newton was as good a Christian as he was a Mathematician & Philosopher. I am
Your most humble servant I: C.