Additional Notes. 4^% 



the upper or lower edge of the spot, and not the mldtlle ancj 

 horizon, on a right Hne, and then subtract or add sixteen im-r 

 nutes for the sun's semi-diameter from or to the zenith dis-^ 

 tance given by the vane. 



N. B. There should he an allowance for the observer's 

 height above the surface of the sea, by subtracting four, five, 

 or six minutes. A table of this kind would not be amiss on 

 the back of die Quadrant. 



There may be some graduations put on the staff, near the 

 centre, to be cut by a plumb-line hung, or a pin put into a 

 small hole for land observations. One of these Quadraats, 

 between eighteen inches and two ittt radius, if v^ell gradu- 

 ated, will be sufficient to take the sun's zenith distance within 

 two or three minutes. 



Succeeding so well with the sun, encouraged me to take 

 what appeared a more difficult task, the finding son^c way to 

 take the altitude of the stars at sea (when the lioiizon may'be 

 seen) better than by the fore-stalF, which 1 concluded must 

 be by bringing the two objects, horizon and star, together. 

 I first considered one reflection ; but the faults of J^avis's 

 Quadrant were here enlarged, which is chiefly the flying of 

 the objects from each other, by the least motion of the in- 

 strument. I then examined wliat two reflections would do, 

 which perfectly answered my desire, being equally useful in 

 taking the distance of stars from each other, and also from 

 the moon, and I believe practicable at sea; for I found that 

 when one star was made to coincide by two reflections with 

 another, the distance of these stars would be double the in- 

 clination of the reflecting planes, as may be easily demon- 

 strated. 



I see but one fault in this instrument, and diat is, that diree 

 feet radius in this has a graduation no larger dian a Quadrant 

 of eighteen inches radius. I hope Dr. Hallet has received 

 a more full account of this from J. Logan, Esq. therefore 

 I shall add no more than that I am, gentlemen, 



Yours, &c. T. GODFREY. 



Philadelphia^ Nov, 9, 1734. 



Page 529. Extracts from " A further Account ofTnoMA^ 

 Godfrey'^ Improvement of Davis'^ Quadrant trans- 

 ferred to the Mariner's Bow.'' 



Being informed diat this improvement, proposed by Tho- 

 mas Godfrey, of this place, for observing the sun's alti- 

 tude at sea with more ease and expedition than is practicable 



