72 Royal Astronomical Society. 



place in the metal at the first surface, there would be no loss, as 

 now, from the thickness of the edges of the glass or the sides of the 

 brass box containing it. 



The second difficulty would be alleviated by the same adoption 

 of the large object-glass : besides the loss of light by transmission 

 through the so-called transparent part of the present horizon-glass 

 would be done away with by the employment of the metal reflectors. 

 The third difficulty was also shown to be gratuitous, for the 

 reflector of the reading-glass was in general so placed that the 

 light of the lamp could not get to it, and if it did, w-ould be thrown 

 away from the arc instead of on it : and were even that managed, 

 the surface of the vernier and arc being in different planes, the 

 same ray of light would not illumine them both at the same time. 

 By placing them, however, both in the same plane, and by putting 

 the reflector at an angle of 45° to the limb, instead of parallel to it, 

 so as to receive parallel light and throw it straight down to the 

 divisions, it was found that they could easily be read by a very 

 faint light. 



For accuracy, opposite readings were deemed essential, and a 

 circle insisted on in place of a sextant or quadrant : and the 

 author, considering that the failure of the reflecting circle in se- 

 curing a permanent footing in the navy arose from its being made 

 in general too large and heavy, and complicated, he had devised a 

 very small, but strong and simple form ; the telescope was mora 

 firmly connected by moving in grooves on the large surface of the 

 face of the circle, instead of rising by the usual single screw ; and 

 in place of the inconvenient plan of having to reverse the hands so 

 as to put the instrument into its box face uppermost (which makes 

 the getting of it out again without pulling at the reflector or some 

 such delicate parts, difficult), by placing the legs not on the back 

 but on the face, the instrument may be either put into its box, 

 or down on the floor, or anywhere, face first, with the same hand 

 which was moving it in the observation, with the divisions and 

 the reflectors protected from all accidents, and the whole instru- 

 ment ready at any time on a moment's notice (for the telescope 

 never need be taken off, with its improved fixing), to take advantage 

 of an instantaneous opportunity of observation. 



So much for the use of the reflecting instrument at sea : as used 

 on land, the following difficulties were found, and are generally re- 

 cognised : — 



1st. The impossibility of measuring in the mercury either sun 

 or star when within, say 20° of the zenith, from the reflecting 

 instrument not taking in so large an angle; and again, when within, 

 say 10° of the horizon, from the foreshortening of the reflecting 

 surface. 



2nd. The difficulty of seeing the referring point all night, viz. 

 the reflected image of the star, when black glass is employed ; and 

 the trouble with wind when employing mercury, as well as with other 

 causes producing vibration ; and the great weight and liability to 

 loss in long journeys through difficult and uncivilized countries. 



All these difficulties seemed to be met by making the reflecting 



