SURVEYING INSTRUMENTS. 77 



of the inventor, Mr. Troughton, contained in a paper which 

 he calls 



Directions for observing with the Reflecting Circle. Pre- 

 pare the instrument for observation by screwing the tele- 

 scope into its place, adjusting the drawer to focus, and the 

 wires parallel to the plane, exactly as you do with a sex- 

 tant ; also set the index forwards to the rough distance of 

 the sun and moon, or moon and star ; and holding the 

 circle by the short handle, direct the telescope to the fainter 

 objects, and make the contact in the usual way. Now 

 read off the degree, minute, and second, by that branch 

 of the index to which the tangent-screw is attached ; also, 

 the minute and second shown by the other two branches ; 

 these give the distance taken on three different sextants ; 

 but as yet, it is only to be considered as half an observa- 

 tion : what remains to be done, is to complete the whole 

 circle, by measuring that angle on the other three sextants. 

 Therefore set the index backwards nearly to the same dis- 

 tance, and reverse the plane of the instrument, by holding 

 it by the opposite handle, and make the contact as above, 

 and read off as before what is shown on the three several 

 branches of the index. The mean of all six is the true 

 apparent distance, corresponding to the mean of the two 

 times at which the observations were made. 



When the objects are seen very distinctly, so that no 

 doubt whatever remains about the contact in both sights 

 being perfect, the above may safely be relied on as a com- 

 plete set ; but if, from the haziness of the air, too much 

 motion, or any other cause, the observations have been 

 rendered doubtful, it will be advisable to make more ; and 

 if, at such times, so many readings should be deemed trou- 

 blesome, six observations and six readings may be con- 

 ducted in the manner following : Take three successive 

 sights forwards, exactly as is done with a sextant ; only 

 take care to read them off on different branches of the 

 index : also make three observations backwards, using the 

 same caution ; a mean of these will be the distance required. 

 When the number of sights taken forwards and backwards 

 is unequal, a mean between the means of those taken back- 

 wards and those taken forwards will be the true angle. 



It need hardly be mentioned, that the shades, or dark- 

 glasses, apply like those of a sextant, for making the objects 

 nearly of the same brightness ; but it must be insisted on, 



