318 SURVEY OF THE COAST 



the telescope and level, and the reading arms are all left to 

 their natuial spring. By this arrangement also, the assis- 

 tant, who sets the level during the ol)servation. acting in a 

 more convenient position, is less liable to affect the position 

 of the instrument by the weight of his hands ; and there be- 

 ing milled heads at both ends of the tangent screw, as in all 

 others of the same kind, he can act with great steadiness by 

 applying both hands. 



The good quality and power of the telescopes is a desirable 

 requisite in the observations ; for it will be found that a great 

 magnifying power facilitates all the observations. There- 

 fore, though the circles are eighteen inches, the telescopes 

 are twenty-two inches, which the instrument bears very well ; 

 and I found that a j)ower of about sixty-six times was tlie 

 most advantageous for use. The object ends have light 

 lengthening lubes of about three inches, adapted instead of 

 the covers, and equilibrating the telescopes. 



The front telescope has three horizontal and three vertical 

 cobweb threads in the focus. The back telescope has only 

 two crossing each other at right angles. 



The four readings of tlie front telescope are marked in tiie 

 order of the divisions by the letters D, E, F, G, whicli serve 

 to register them properly in the journals of oI)servations, in 

 the same manner as those of the back telescope are repre- 

 sented by W, X, Y, Z. 



As a very great convenience for night observations, Mr. 

 Troughton usually adds to his circles a smaller divided cir- 

 cle, fixed to the part bearing the level, and to the telescope 

 he adds an arm, reaching over the frame of the instrument 

 to this circle. Sliding pieces are adapted to this circle, and 

 being adjusted to the proper zenith distance of the star, on 

 both sides of the zenith, will arrest the telescope in such a 

 situation as to bring the star in the field of the telescope, by 

 the horizontal revolution of the instrument. Tiiis arrange- 

 ment underwent a shght modification, on account of the in- 

 terruption which a long arm from the front telescope would 

 have met with from the back alhidade. 



