[fol. 69] Paris feb. the 21th Sr I entirely rely upon yr goodness to be pardon’d the liberty I take in giveing you this trouble which proceeds from a great ambition I’ve had this good While of communicating with the royal Society some Mechanical principles which lead me into an Essay concerning the Mechanical Causes of gravity, elasticity, and of the cohesion or Mutual attraction of the primary insensible parts of bodies, as alsoe of the Mutual aversion or repulse of others, as is observable by phenomena. for the undertaking this essay, I had found out and Demonstrated from principles universally receiv’d (at least by the royal Society of London) a law of Nature, whereby a rare elastick Medium (such as the Incomparable Sir Isaac Newton’s Other) Must act upon all bodies, not as all other Mechanical Causes hitherto observ’d, according to the quantity of surfaces, but constantly according to the quantity of solid matter contain’d in the bodies, and in their Insensible parts on wch such a rare elastick Medium Acts, without any regard to the quantity of surfaces; and Acts thus upon all bodies at considerable distances which are always proportionable to the solid Magnitudes of the bodies. for (tho the philosophers of this wide of the world don’t seem to be aware of it) it is well known from phenomena especially that of the equal accelaration of all sorts of bodies falling in Vacuo Boyleano, that without having first demonstrated such a Mechanical law as this I’ve talked of ’tis in Vain to seek for a Mechanical Cause of gravity; I mean any cause consisting in any exteriour action or impulse of any kind of Matter towards my common point such as the Center or rather focus or umbilicus of a system of bodies Moveing in curves about said focus. And from this Law, Whereby any rare elastick Medium Must thus act upon bodies, I am in hopes may be found to proceed them vires Nature of which the great Newton speaks in he preface of his Mathl principles; beginning with this remarkable phrase, Multa me Movent ut nonnilul suspicer & exquibus viribus ignotis (says he) philosophi hactenus Naturam frustra lentarant [?] Besides explaining those Vires Nature (residing in Newton’s spiritus subtilissimus, pag. ult. princip. Mot., iujas vi et actio uibus, says he ibid., perticule corporum ad Minimas distantias se Mutuo attrahuat et antigue facte inherent &c) [?] I have Endeavour’d to deduce from phenomena and known Me chemical Laws, the true constitution of all systems of bodies Moving in Curves about one common focus or umbilicus, in which constitution I’ve found to appear Most Naturally the vis centripeta acting by a Constant Mechanical Law towards the Common focus of the system And I must beg leave to observe that this discovery (if not unknown to me, ill grounded) Must be of the last importance to Natural philosophy this I am sure that My attempt is very Candid and down right bona fide, nor do I think to have rely’d upon any one supposition in all my essay, but altogether upon known phenomena, and [fol. 70] receivd Mechanical principles tho’ not hitherto apply’d in the same Manner that I have by Chance apply’d them. I have alsoe explain’d according to Mechanical principles depending on the discover’d Mechanical cause of gravity the compounded forces whereby all Circumsolar bodies ascend from their perihelia tot he Aphelia and vice versa, describing about the common focus ellipses more or less Eccentrick according to the various ways whereby those compounded forces act on upon another; which various ways and laws of acting, and what they proceed from I undertake to explain in Mixt Mathematicks from phenomena & known mechanical Laws. and do think (at least as far as I can convince my self) to have assign’d the true mechanical cause of that more or less eccentricity of all Elliptick orbits; as alsoe to have given a compleat Mechanical theory of all the phenomena of Comets as they are collected and observed by the most admirable industry of the great Newton and Halley. In short all their celestial phenomena that Sir Isaac Newton explain’d by his own Mathematical word Attraction in the Corallaries of his 36th prop. book 1. of which attraction he says in the entrance of that section pag. 147 quamvis [?] for laste, si physice loquamar, verius dicatur impulses; [?] I say all them phenomena I endeavour so to explain as to assign the mechanical and physical Cause of that attraction, consisting in or proceeding from either the simple mechanical cause of gravity already assign’d, or else in or from the compounded forces of gravity towards the focus and of the first project […] immechanical motion impress’d on the Celestial bodies by the first Imechanical Cause to which from the same principles I’ve added the physico-mechanical Causes of the attraction or gravitation of over seas towards the bodies of the Moon & Sun and Sun [sic]; and of the perturbation or Mutual attraction of the planets sensibly observable in the conjunction of Jupiter and Saturn so that I think to have assign’d and Explain’d the true Mechanical Cause of all sensible general phenomena observable in our system consisting either in the simple force of gravity towards the focus, or in the foremention’d forces compounded therewith but of the action of the Cause of gravity towards the said focus i.e. of the centripetal action of Newtons spiritus subtilissimus or Medium other[…] (whereof pag. ult. princip. and quest. 21 ad fin. opt.) I say of the Centripetal action of this Medium I don’t think there is any Mechanical exteriour cause to be assign’d but the first constitution or Laws of the great Imechanical Cause; and there a natural philosopher must rest and give up his Mechanical Causes,and he may call that first imechanical constitution or Law whereby the other acts towards the Center, he may call it, I say, attraction or whatever Name he thinks fit[.] I have ventur’d, sir, to tire your patience thus for with this sort of Memoire, and shall beg the favour of your protection, if you find there possibly cou’d be any expectation of succeeding upon such an attempt as I here describe; of which I am sensible there may be great doubt considering many observations lately made in Natural philosophy whereby the Cause of gravity seems to be no way Mechanical but as Sir Isaac Newton himself was of opinion [sic] it may be mechanical, and as the know of the difficulty coud act upon bodies according to the quantity […] Mattr which I think I have explain’d, I therefore woud be glad to push my attempt as far as the royal Society wou’d find it consistent with true principles; and I am in some hopes that the royal Society of London (above all others) may […] my principles not insistent wth theirs. Wherefore the least Incouragemt from you sir woud Make me go over to London for some Months to Lay down My essay at yr feet & to assure you how much I am your Most obedient humble servt J. Brown My address […] is Chez Mr Loftus banquier anglois A Paris [fol. 71] I beg leave Sir to give you My address at large thus A Monsieur, Monsieur Brown recommendé a Monsr Loftus banquier Anglois A Paris in case you favour me with an Answer Mr. Browns letter paris feb 21 1731
Brown discusses his physical studies and the work of Sir Isaac Newton in great detail.
