VOL. LXXIV.] PHILOSOPHICAL TRANSACTIONS. 517 



labour as at the first. Equatorial sectors are in the hands of few ; and require 

 great skill. Some more general method seemed wanting ; to discover variations 

 which, when detected or only surmised, should be consigned immediately to a 

 more strict investigation. 



Turning this in my thoughts, I considered, that the noting down at the time 

 the exact appearance of what one sees, would be far more simple, and show any 

 alterations in that appearance more readily, than any other method. A drawing 

 once made would remain, and could be consulted at any future period ; and if it 

 were drawn at first with care, a transient review would discover to one, whether 

 any sensible change had taken place since it was last examined. Catalogues, or 

 verbal descriptions of any kind, could not answer that end so well. To do this 

 with ease and expedition was then the requisite: and a telescope with a large field, 

 and some proper sub-divisions in it, to direct the eye and assist the judgment, 

 seemed to bid most fair for success. The following is the method which, after 

 various trials, I have adopted, and think I may now venture to recommend. 



To a night-glass, but of Dollond's improved construction, which magnifies 

 about 6 times, and takes in a field of just about as many degrees of a great cir- 

 cle, I have added cross wires, intersecting each other at an angle of 45°. More 

 wires may be crossed in other directions ; but I apprehend these will be found 

 sufficient. This telescope I mount on a polar axis. One coarsely made, and with- 

 out any divisions on its circle of declination, will answer this purpose, since there 

 is no great occasion for accuracy in that respect : but as the heavenly bodies are 

 more readily followed by an equatorial motion of the telescope, so their relative 

 positions are much more easily discerned when they are looked at constantly as in 

 the same direction. An horizontal motion, except in the meridian, would be 

 apt to mislead the judgment. It is scarcely necessary to add, that the wires must 

 stand so as for one to describe a parallel of the equator nearly. Another will 

 then be a horary circle ; and the whole area will be divided into 8 equal sectors. 



Thus prepared, the telescope is to be pointed to a known star, which is to be 

 brought into the centre or common intersection of all the wires. The relative 

 positions of such other stars as appear within the field, are to be judged-of by the 

 eye : whether at -L, or x, or 4- from the centre towards the circumference, or 

 vice versa ; and so with regard to the nearest wire respectively. These, as one 

 sees them, are to be noted down with a black lead pencil on a large message card 

 held in the hand, on which a circle, similarly divided, is ready drawn. (One of 

 3 inches diameter seems most convenient.) The motion of the heavenly bodies 

 in such a telescope is so slow, and the noting down of the stars so quickly done, 

 that there is most commonly full time for it without moving the telescope. When 

 that is wanted, the principal star is easily brought back again into the centre of 

 the field at pleasure, and the work resumed. After a little practice, it is astonish- 



