requisite circumstances : but as neither Mr. Ludlam, nor 

 any other person that I know, has explained the grounds of 

 the directions he has given ; and as these directions will pro- 

 bably be applied in an unskilful and negligent manner, un- 

 less it be generally understood and impressed of what im- 

 portance they are : as moreover this is the method, at least 

 the most generally practicable, of adjusting the back hor". 

 glass, as well as of trying the accuracy of the construc- 

 tion of the quadrant for effectiug it in Mr. Blair's method ; 

 and is also subservient to the contrivance hereafter men- 

 tioned for accomplishing it in the same Avay ; it is necessary, 

 before I proceed to the description of it, to state the prin- 

 ciples on which Mr. Ludlams judicious instructions are 

 founded. 



He directs that the back horizon glass may be adjusted at 

 i-ight angles to the index glass, by the means of two near 

 objects, such as two lines sustaining plummets in water, 

 or two candles *, &c. lying in the plane of the quadrant 

 placed horizontal, and in a line joining the objects equidist- 

 ant from the quad'.; one of them being before, and the 

 other behind the observer; by reversing the instrument by 

 'turning it half round in its own plane, and shifting its posi- 

 tion laterally on cither sidc^ till the images of the two objects 

 are seen, through the back sight vane, to coincide, when 

 each of them alternately is viewed by the observer, by di- 



• WTien plummets are used, they must bo placed at opposite doors or windows against 

 the light of the sky: and if candles be employed, their light should he seen tlirough a 

 small slit in a screen placed before each. 



