^4S Proofs from old English Books, that the 



on the passages before us, by contemplating the very great 

 distance at which Digges asserts that his father fired gun^ 

 powder by means of the solar rays ? a distance which ex- 

 ceeds that which, in our former letter, we ventured, per- 

 haps hastily, to think rather poetical, in a greater ratio than 

 that of 2640 (the feet in half a mile) to 2002, or than that 

 of 13 to 10. The gentlemen could not be ignorant that 

 Archimedes was said to have produced his famous confla- 



f ration at a great distance ; and this, after all, may have 

 een the true state of the fact. But it is probable that 

 neither of them had seen Napier's proposal to excite flame 

 '■' at any appointed distance." Surely an expression which, 

 as observed in my last, needs qualification, cannot be ad- 

 duced in support of another which almost exceeds belief. 

 But, to avoid all controversy in a case which admits of 

 some latitude of opinion, I would respectfully ask better 

 judges of the subject. Whether the light * of the sun, re- 

 flected from terrestrial objects, be not always accompanied 

 with a real, though often insensible heat, which aeteris pa- 

 ribus, is ever proportional to the density of such light? 

 Whether windows partially reflecting the rays of the sun 

 when near the horizon, may not be seen at a great distance ; 

 and whether the spectator does not receive from them a real, 

 though generally an immeasurably small, portion of heat ? 

 Whether the sun-beams, partially reflected from the smooth 

 surface of the sea, do not sensibly increase the heat on 

 shore ; and whether this effect be not miiversally experi- 



* Sn rnuch convinced was the celebrated Gravesande of the intimate 

 coniuction between hre and light, that he defines a lucid body in these 

 words — " Corpus 'vacatur li(c:dum, (juod lumnt rmittit, id en, jgnevi per li- 

 nfai rectus agitat. A body is called lucid which emits l/ght, that is, 

 gives fire a motioti in right lines." Pfys. Elem. maihem, Exper. coifcru 

 printed 1711, vol. ii. p. 11. See also the able Dr. Hutton's Dictionarv, 

 article L:ght. For the benefit of those wlio are alwaj'S hunting after 

 new books, which are very often old books improved for the worse, it may 

 not be amiss to arid that, about seventeen years ago, I heard professor 

 John Robison. of E'linburgh, certainly one of the best judges in Europe, 

 recommend this work of Gravesande (the third edition, if I rightly re- 

 member) as, upon the whole, the best book of the kind that had then 

 been published ; and I do not know that any work of equal merit and 

 extent has since appeared. Whether the professor said any thing in 

 prjise of the English translation I do not recollect, but I rather think 

 not. The truth is. that the excellent preliminary discourse, the only paft 

 ^f th" English edition I have read, is but indifferently translated. A 

 work on natural philosophy has lojig been expected from Mr. Robisoa 

 himself; but science has not hitherto received this bcneht, owing, I be- 

 lieve, to his poor state of health. We may judge of tlie manner in 

 v«.hich it would have been executed by the articles he has given to the 

 Encyclopaedia Britannica. 



enced^ 



