VOL. XIV.] PHILOSOPHICAL TRANSACTIONS. JQ 



as useful as ordinary salt is. It tinges all the stones of a red colour. The ori- 

 ginal of this spring cannot be fetched any farther off than the rock out of 

 which it issues; for the sea, which is the great treasure of salt water, is at too 

 great a distance to dispense any streams to this place; being 8 miles off, where 

 nearest. 



Exercitatio Geomelrica de Dlmensione Figurarum, Author e Davide Gregorio,* in 

 uicademia Edinburgensi Matheseos Professore. Edinburgi in Ato. l684. 

 N° 163, p. 730. 

 In this work the author first takes notice of a treatise of his uncle, Mr. 



* This David Gregory was the nephew of the celebrated James G. inventor of the telescope of 

 his name, and a worthy inheritor of his mathematical genius ; a genius which has run through the 

 family of Anderson and Gregory for several generations. Indeed it is a singular fact that our author, 

 David Gregory, and two of his brothers, James and Charles, were all mathematical professors at 

 the same time in three British universities, viz. David at Oxford, James at Edinburgh, and Charles at 

 St. Andrews. David, the subject of the present memoir, was first elected to the mathematical chair 

 at Edinburgh in l684, being then only 23 years of age, and in the same year he published the book 

 above noticed. In 1691, on the resignation of Dr. Bernard, he was removed to the astronomical 

 chair at Oxford, being recommended and befriended on that occasion by Sir I. Newton and Mr. Flam- 

 steed, in opposition to their friend Mr. Halley; who was rejected, as was said, on account of his 

 sceptical principles. In this situation Dr. Gregory rendered great service to the mathematical sci- 

 ences by several important works. As, some valuable communications^ the Royal Society, pub- 

 lished in their Transactions, vols. 18, 19, 21, 24, 25. In 1695, he printed at Oxford his Catoptricae 

 et Dioptricae Sphericae Elementa, being the substance of some lectures read at Edinburgh j and the 

 same was afterwards twice published in English, by Dr. Brown, and by Dr. Desaguliers, with the 

 addition of an account of the Newtonian and Gregorian telescopes, &c. It is not unworthy of remark 

 that, in the conclusion of this treatise is found an obser\'ation, which shows that the construction of 

 achromatic telescopes, which Mr. Dollond has carried to such perfection, had occurred to the mind 

 of David Gregory, from reflecting on the admirable contrivance of nature, in combining the different 

 humours of the eye. The passage is as follows : " Perhaps it would be of service to make the object 

 lens of a different medium, as we see done in the fabric of the eye; where the crystalline humour 

 (whose power of reflecting the rays of light differs very little from that of glass) is by nature, which - 

 never does any thing in vain, joined with the aqueous and vitreous humours (not differing from water 

 as to their power of refraction) in order that the image may be painted as distinct as possible on tlie 

 bottom of the eye." The master-piece of Dr. Gregory, however, is his Astronomicae Physicae et 

 Geometricae Elementa, published in folio at Oxford, 1702. This work is founded on the Newtonian 

 doctrines, and was esteemed by Newton himself as a most excellent explanation and defence of his 

 philosophy. The following year Dr. Gregory gave to the world an edition, in folio, of the whole 

 works of Euclid, in Greek and Latin; being done in prosecution of a design enjoined by the founder. 

 Sir Harry Saville, of publishing the works of all the ancient mathematicians. In prosecution of the 

 same plan, our author engaged soon after, witli his colleague Dr. Halley, who had been elected the 

 geometry professor, in the publication of the conies of Apollonius, when his useful labours were ar- 

 rested by the hand of death, in 1710, being only the 49th year of his age. 



Besides those works published in our author's life-time, as above-mentioned, he lefl also in mana- 



