Additiojud Notes. 47 1 



ments, applied himself to tiie study of that language, till he 

 could pretty well understand an author on these suhjects; after 

 which, the first time 1 ever saw or heard of him, to my 

 knowledge, he came to borrow Sir Isaac Newton's Prin- 

 cipia of me. Inquiring of him hereupon who he was, I was 

 indeed astonished at his request; but after a little discourse, he 

 soon became welcome to that or any other book I had. 'I'his 

 young man, about eighteen months since, told me he had 

 for some time been thinking of an instrument for taking the 

 distances of stars by reflecting speculums, which he believed 

 might be of service at sea; and not long after he showed me 

 a common sea Quadrant, to which he had fitted two pieces 

 of looking-glass in such a manner as brought two stars, at 

 almost any distance, to coincide ; the one by a direct, the 

 other by a reflected ray, so that the eye could take them both 

 together as joined in one, while a moving label or index o» 

 the graduated arch marked exactly half their distance: fori 

 need not say that the variations of the angles of reflection 

 from two speculums are double to the angle of the inclination 

 of their planes, and therefore gives but half tJie angle or 

 arch of the distance, which is the only inconveniency that 

 appears to me to attend this. But as it may be made so sim- 

 ple, easy and light, as not to be much more unwieldy or 

 unmanageable, though of a considerable length, than a single 

 telescope of the same, that inconveniency will be abundantly 

 compensated. 



The description of it, as he proposes it, and has got one 

 made, is nearly thus, which he is willing I should communis 

 cate to thee, if possibly it may be of service. 



To a straight ruler or piece of wood, A B, of about three 

 inches in breadth, and from 40 to 45 in length (or of any 

 other that may be thought convenient), with a suitable thick- 

 ness, an arch or limb, A C, of about 30 degrees to the ra- 

 dius, K L, is to be fixed. To the upper end of the piece 

 A B, a piece, D D, is to be morticed, and in it the centre 

 K taken, so that O P may be about six inches, and the angle 

 K O P about 40 degrees. On this centre K, the ruler or in- 

 dex K L is to move, having a fiducial edge below answer- 

 able to the central point, to cut the graduations on the limb. 

 On the upper end of this index a speculum of silvered glass, 

 or rather metal, exacdy plain, E F, of about three inches in 

 length and two in height, is erected perpcndi(^ular to the 

 plane of the index, and also nearly at right anglt^s with its 

 sides, the plane of the reflecting surface standing exactly over 



