you may yourself, in those parts where you are, concerning natural and artificial things, but also of your interest in the other English plantations, for exciting them to join in the same with you. Which being so, I am particularly to recommend to you and your ingenious friends in America, that you would impart to the Royal Society whatever, in the Bermudas and the other colonies, occurs considerable for the enriching of the History of Nature, (a faithful composure whereof is one of the main things they have in their eye,) and more especially of the history of the tides, the particulars whereof, if well observed about such islands, as lie in the open ocean, (the Bermudas, St. Helena,) would probably give much light for the finding out of a good theory to solve those puzzling phænomena, that occur about that subject. Besides, we being informed of a new whale-fishing, undertaken about the Bermudas 1, we should be very glad to receive from you the truth, method, and success of that enterprise, with a description of the kind and qualities of those whales, and whether any of that substance, called sperma ceti, be found in them, and if so, in what part and quantity, and how 'tis ordered: to which if you would please to add what observations you make of eclipses, of the motions of the satellites of Jupiter, and such like, you would thereby exceedingly oblige the public, and gratify the Royal Society, and particularly, Sir,

h

your very humble servant,

H. OLDENBURG, Soc. R. Secr.

P.S. I send hereby inclosed two of those tracts, called Philosophical Transactions, which are here printed once a month, wherein you will find, in the directions h See Phil. Trans. vol. i. p. 11, 132.

for seamen, and in the inquiries and considerations of Sir Robert Morayk and Dr. Wallis', what the particulars are we desire chiefly to be informed about, in the matter of the tides. Sir, what you shall think fit to send of this nature for the Royal Society, if you please only to address it to me, as one of their secretaries, living in the middle of the Pall Mall of St. James's Fields in Westminster, will safely come to their hands, if the ship that brings it, comes safe to port.

Sir, we are informed that you have by you many accurate maps, as well of other parts of the world as of America, and of the particular plantations of the English, Dutch, &c. Our request is that they may be carefully preserved; and, if you would please to lodge them in the repository, or with the books and writings of the Royal Society, they would receive them as a singular testimony of your respect and affection to them; as also, if you should consent to the publishing thereof, [they would] cause them to be printed as yours, as indeed they are, with a character due to your person and merits.

No. XVII.

NEWTON ON THE CAUSE OF GRAVITATION.

1st. I suppose that there is diffused through all places an ethereal substance, capable of contraction

i No. 8, p. 140; No. 9, p. 147. k No. 17, p. 298. There were other papers of Sir R. Moray on the tides (No. 4, p. 53; and No. 18, p. 311).

1 No. 17, p. 297. Wallis had likewise published on the same

subject in No. 16, p. 263, 281; but 8 and 17 were most probably the two numbers which were sent, with his letter, by Oldenburg.

m From his letter to Boyle. Horsley's Newton, vol. iv. p.385.

and dilatation, strongly elastic, and, in a word, much like air in all respects, but far more subtile.

2. I suppose this ether pervades all gross bodies, but yet so as to stand rarer in their pores than in free spaces; and so much the rarer, as their pores are less. *

*

3. I suppose the rarer ether within bodies, and the denser without them, not to be terminated in a mathematical superficies, but to grow gradually into one another the external ether beginning to grow rarer, and the internal to grow denser, at some little distance from the superficies of the body, and running through all intermediate degrees of density in the intermediate spaces.

4. When two bodies, moving towards one another, come near together, I suppose the ether between them to grow rarer than before, and the spaces of its graduated rarity to extend further from the superficies of the bodies towards one another; and this by reason that the ether cannot move and play up and down so freely in the straight passage between the bodies, as it could before they came so near together.

5. Now from the fourth supposition, it follows, that when two bodies, approaching one another, come so near together as to make the ether between them begin to rarify, they will begin to have a reluctance from being brought nearer together, and an endeavour to recede from one another: which reluctance and endeavour will increase as they come nearer together, because thereby they cause the interjacent ether to rarify more and more: but at length, when they come so near together, that the excess of pressure of the external ether, which surrounds the bodies, above that of the rarified ether, which is between them, is so

great, as to overcome the reluctance, which the bodies have from being brought together, then will that excess of pressure drive them with violence together, and make them adhere strongly to one another, as was said in the second supposition. *

I shall set down one conjecture more, which came into my mind now as I was writing this letter it is about the cause of gravity. For this end, I will suppose ether to consist of parts differing from one another in subtilty by indefinite degrees: that in the pores of bodies, there is less of the grosser ether in proportion to the finer, than in open spaces; and consequently, that in the great body of the earth there is much less of the grosser ether in proportion to the finer, than in the regions of the air: and that yet the grosser ether in the air affects the upper regions of the earth, and the finer ether in the earth the lower regions of the air in such a manner, that, from the top of the air to the surface of the earth, and again from the surface of the earth to the centre thereof, the ether is insensibly finer and finer. Imagine now any body suspended in the air, or lying on the earth; and the ether being, by the hypothesis, grosser in the pores which are in the upper parts of the body, than in those which are in the lower parts; and that grosser ether, being less apt to be lodged in those pores, than the finer ether below; it will endeavour to get out, and give way to the finer ether below, which cannot be, without the body's descending to make room above for it to go out into.

The original of this letter is in the collection of the Earl of Macclesfield: and the above extracts have been corrected by it.

No. XVIII.

HUGENS ON THE CAUSE OF GRAVITATIONo.

P. 135. Pour expliquer donc la pesanteur de la manière que je la conçois, je supposerai que dans l'espace sphérique, qui comprend la terre et les corps qui sont autour d'elle jusqu'à une grande étendue, il y a une matiere fluide qui consiste en des parties très petites, et qui est diversement agitée en tous sens, avec beaucoup de rapidité. Laquelle matiere, ne pouvant sortir de cet espace, qui est entouré d'autres corps, je dis que son mouvement doit devenir en partie circulaire autour du centre; non pas tellement pourtant qu'elle vienne à tourner toute d'un même sens, mais en sorte que la plupart de ses mouvemens différens se fassent dans des surfaces sphériques, à l'entour du centre du dit espace, qui pour cela devient aussi le centre de la terre.

La raison de ce mouvement circulaire est que la matière, contenue dans quelque espace, se meut plus aisément de cette manière que, par des mouvemens droits contraires les uns aux autres, lesquels même en se refléchissant, (parceque la matière ne peut pas sortir de l'espace qui l'enferme,) sont reduits à se changer en circulaires.

P. 137. Si parmi la matière fluide, qui tourne dans l'espace que nous avons supposé, il se rencontre des parties beaucoup plus grosses que celles qui la composent, ou des corps faits d'un amas de petites parties accrochées ensemble, et que ces corps ne suivent pas le mouvement rapide de la dite matière, ils seront nécessairement poussés vers le centre du mouvement, o From Hugens' Discours de la Cause de la Pesanteur. APPEND.

F