I have read the first Chapter of Dr. Colden’s piece of Gravitation which you was so good as to inclose in yours of the 14th. Ult. and endeavoured to understand it, but with as little Success as yourself. I have ventured notwitstanding to make in the margin a few remarks on several passages, which detach’d from the rest I tho’t I understood; but it is more than probable they are very impertinent when those passages are considered in the relation they stand in to the whole: and in that relation I acknowledge they ought to be consider’d. As to the three principles he endeavours to establish which he says are all agents essentially different from each other and will serve to explain all the phaenomena of Gravitation, I wish for my own sake he had produced more evidence of their existence than he has. As to the first viz. Resisting matter which resists in all directions, but in the conception of which motion ought not to enter; it may easily be allowed him with some limitation: but the other two viz. Self-moving matter which is void of all resistance, and exerts its power only in one direction; And Elastic matter which has neither resistance nor motion, nor exerts any action independent of the other powers, but being the medium of conveying their actions, receives them in one direction and reflects them in all directions; are not easily understood, and require more exemplication than is to be found in the treatise. But allowing these principles to exist, I am unable to explain any phaenomenon of Gravitation by them. I have applied them particularly to the revolutions of the planets round their centre of Gravity: Thus supposing all the planets like the Earth to be resisting matter and impell’d immediately by the self-moving matter, they would only have a direct motion communicated to them; or if the Self-moving matter acted on the elastic matter, and that convey’d it’s action to the planets, the elastic matter would give or convey to them only a direct or rectilinear motion: for tho’ the elastic matter is said to reflect the received action in every direction, that action must be in right lines in every direction, or it would not be the action of the self-moving matter (which is exerted in one direction only and consequently in a right line) communicated, &c.

But supposing these principles sufficient (and I ought to suppose them sufficient till I have seen the Drs. Application of them) to solve the phaenomena of Gravitation, it will be necessary for him I conceive in order to have a full conception of the Cause of Gravitation, to enquire by what means those principles had existence; and how they became endowed with their respective powers. This seems an arduous task on mechanical principles. I should be glad to see the remainder of the Drs. Piece and the new Edition of it lately published in England (by Mr. Dodsley 4to.) which when you have opportunity I shall take it as a favour if you’d send me.

The Drs. Book and Letter I now return, for which I am obliged to you and am most respectfully Sir Yours.