TOL. LXII.^ PHILOSOPHICAL TRANSACTIONS. 2^1 



Xlf^. A Letter from Mr. Peter Dol/ond, describing some Additions and Altera- 

 tions made to Hadleys Quadrant, to render it more Serviceable at Sea. p. 95. 

 The glasses of the Hadley's quadrant should have their two surfaces perfect 

 planes, and perfectly parallel to each other. From several years practice in 

 grinding these glasses, I have found out methods of making them to great ex- 

 actness; but the advantage, that should arise from the goodness of the glasses, 

 has often been defeated by the index glass being bent by the brass frame that 

 contains it: to prevent this, I have contrived the frame, so that the glass lies on 

 3 points, and the part that presses against the front of the glass has also 3 points 

 exactly opposite to the former. These points are made to confine the glass by 3 

 screws at the back, that act exactly opposite to the points between which the 

 glass is placed. This little contrivance may be of some use; but the principal 

 improvements are in the methods of adjusting the glasses, particularly for the 

 back observation. 



The method hitherto practised for adjusting that part of the instrument, by 

 means of the opposite horizons at sea, has been attended with so many diffi- 

 culties, that it has hardly ever been used; for so little dependance could be made 

 on the observations taken this way, that the best Hadley's sextants made for the 

 purposes of observing the distances of the moon from the sun or fixed stars, 

 have been always made without the horizon glass for the back observation; for 

 want of which, many valuable observations of the sun and moon have been lost, 

 when their distance has exceeded 120 degrees. , 



To make the adjustment of the back observation easy and exact, I have ap- 

 plied an index to the back horizon glass, by which it may be moved into a pa- 

 rallel position to the index glass, in order to give it the two adjustments, in the 

 same manner as the fore horizon glass is adjusted. Then, by moving the index, 

 to which the back horizon glass is fixed, exactly gO degrees (which is known by 

 the divisions made for that purpose) the glass will be set at right angles to the 

 index glass, and consequently will be properly adjusted for use, and the observa- 

 tions may be made with the same accuracy by this, as by the fore observation. 



To adjust the horizon glasses in the perpendicular position to the plane of the 

 instrument, I have contrived to move each of them by a single screw, that goes 

 through the frame of the quadrant, and is turned by means of a milled head at 

 the back, which may be done by the observer while he is looking at the object. 

 To these improvements I have added Mr. Maskelyne's method of placing 

 darkening glasses behind the horizon glasses. These glasses, which serve for 

 darkening the object seen by direct vision, in adjusting the instrument by the 

 sun or moon, I have placed in such a manner, as to be turned behind the fore 

 horizon glass, or behind the back horizon glass, that they may be used with 

 either; there are 3 of these glasses of different degrees of darkness; the lightest 



p p 2 



