OF THE UNITED STATES. aUl 



favourable circumstances of temperature, always exceeding- 

 ly correct, and rivalling the large upper level, wlien tiiat 

 nicety of adjustment was observed which their smaller scale 

 naturally required. 



Tlie transit telescope, which forms the upper part of this 

 instrument and by which the angles are observed, is sup- 

 ported by two columns fifteen inches high screwed upon the 

 drum at right angles to the microscope A. In travelling, 

 these colunuis are unscrewed at oo, to prevent the box in 

 which they are carried from being top heavy, and more ef- 

 fectually to secure them from injury ; though I had the box 

 also fitted to receive them with the instrument in some 

 cases, in order not to disturb the upper adjustments of the 

 instrument. 



>t the lower part of the drum, in the direction of the two 

 columns, towards flie right from the microscope A, is the 

 arm k, made broad, but thin, so as to have sufficient strength 

 in the horizontil direction and yet very little friction on the 

 limb. This has the clamping and tangeni screws, and pre- 

 sents itself always conveniently to the hand, in all positions 

 of the instrument towards the observer. 



The tops of the two vertical columns bear two pieces //jy, 

 projecting outwards to admit the axis, twelve inches in length, 

 of the transit telescope, which is supported between them in 

 rectangular Y's as usual. In one of these pieces, at ;•, is a 

 screw, with a capstan head, showing through a cut in the 

 side, which bears by the head upwards against the piece ;/, 

 and by the screw part below against the uppermost part of 

 the column, forcing the piece y upwards by the mere spring 

 of the metal, and, between this bearing point and the screw 

 5, fastening this piece to the column. 



The telescope is a complete and very excellent transit 

 instrument, describing a whole vertical, the eye end of the 

 telescope havitig room to pass between the two columns 

 witliout touching the top of the axis /^'. It has thirty inches 

 focal length, two inches and a hall' apeiturc, and four numni- 

 fying powers, the largest of which is about sevcnlv-scven 



