IMPROVED REFLECTING CIRCLE. 1 1 J» 



possible time, which cannot be effected in near so short a time 

 by screens which take off and on, as in Borda's reflecting 

 circle. 



Captain Mackay, who has commanded the Lord Forbes, in the 

 Jamaica trade, for twenty years, stated, that the manner in 

 which Mr. Allan's screens are fixed in his reflecting circle is a 

 great improvement. That from not being obliged to take out 

 the shades when the sun is clouded, the object is not lost ; and 

 that when an instrument is obliged to be taken from the eye, 

 to fix the screen in the old mode, the object is Id^t. 



That by this instrument being a reflecting circle, it makes 

 sure of a horizontal plane well divided, which can hardly be 

 the case in a portion only of a circle. 



That Mr. Allan's is the most complete instrument he has 

 ever seea, and that he shall always take one with him to 



Description of the Drawing of Mr. James Allan'' s Improve- 

 ment on the Refecting Circle of Borda. PI. III. 



The reflecting circle, first invented by Tobias Mayer, of Got- description of 

 lingen, and afterwards improved by the chevalier La Borda, Mr.Allan'sim- 

 of Paris, is an instrument, which in its principle admits of PJ?^^j.^[J "^' 

 such a degree of accuracy, as jto be of the most important 

 service to navigators j but it has hitherto been constructed 

 in such a manner, that the inconveniences attending the use 

 of it have prevented its general adoption among seamen : any 

 contrivances, therefore, tending to diminish these inconve- 

 niences, were deserving of the Society's notice. The con- 

 struction of Borda's circle, as it has hitherto been made, 

 is minutely detailed in Dr.Rees's New Cyclopaedia, article. Cir- 

 cle J and the mode of using it is there explained ; it will be 

 therefore unnecessary to describe any thing more of the circle 

 delineated in pi. III, than is essential to the elucidation of the 

 improvements made by Mr. Allan. 



The first of these is in the mode of applying the dark glasses. Application o£ 

 which are fixed on joints, so as to turn back out of the way, the dark gl«M- 

 in the same manner as in the sextant. In the old instrument 

 these glasses were fitted into socket! provided with tenons on 

 the indexes, and fastened by a milled head screw, which took 

 much time to change them. The second is the addition of 

 I 2 double 



