t 97 3 



at right angles to the index glafs, and confequently 

 will be properly adjufted for ufe, and the obfer- 

 vations may be made with the fame accuracy by 

 this, as by the fore obfervation. 



To adjuft the horizon glafTes in the perpendicular 

 pofition to the plane of the instrument,- I have con- 

 trived to move each of them by a fingle fcrew, 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 obferver while he is looking at the 

 object. 



To thefe improvements, Sir, I have added your 

 method of placing darkening glafTes behind the ho- 

 rizon glafTes, which you have been fo kind as to give 

 me liberty to apply to my inftruments. Thefe 

 glafTes, which ferve for darkening the object feen by 

 direct vifion, in adjufting the inftrument by the Sua 

 or Moon, I have placed in fuch a manner as to be 

 turned behind the fore horizon glafs, or behind the 

 back horizon glafs, that they may be ufed with 

 either; there are three of thefe glafTes of different 

 degrees of darknefs ; the lightest or paleft I do 

 imagine will be of ufe in taking the Sun's altitude 

 when the horizon appears glaring, which I believe 

 often happens by the reflection of the fea. 



If thefe additions and alterations mould be thought 

 to be real improvements, which I cannot doubt, Sir B 

 if they are honoured with your approbation, I hope 

 they may ferve in conjunction with thofe improve- 

 ments you have made yourfelf in refpect to the ob* 

 viating any poflible errors in the parallelifm of the 

 planes of the index glafs, and in regard to the ad- 

 justment of the telefcope parallel to -the plane of the 



V-o-l.JLXII. O quadrant* 



