OF THE UNITED STATES. 281 



ihe centre, as seen in Tlatc IV. A ninnl)eroftIiesc tripods, 

 ?liftk'icnt to sujiport two Ien2;lh-i of l)ais. niust he in icadi- 

 ness. They should have lej;s of dilliTent lcn>j;th'«. so that 

 they may be raised or lowered, to suit the inequahtics of the 

 ground. 



'I'o place these tripods properly, so that tlie l)o\ may rest 

 equally on them, it is iiecessaiy to have a lari^jc level, similar 

 to those used by masons, and lotis; enouG;h to reach three tri- 

 pods. It should have an alhidade han.2;ins vertically l)y its 

 own weight, with a horizontal spirit level, and an arc Ixdow 

 consisting of as many degrees as the locality of the ground 

 may require, as seen at Plate IV. tig. 3. 



The hox includina; the bars and the other apparatus de- 

 s( r-l)cd. of which XX is the bottom, is about nini* inehes in 

 \\id;h and lie'glit at l)oth ends and llfleen inches in the n)id- 

 (llc. it is strengthened at the upper part l)y seven crossing 

 pieces (Id, with notches holding tlie sides in their [)roper 

 jiositions. There is no wooden cover to the box, as a piece 

 of canvas is intended to be huns; loosely over it, vvhicli pre- 

 vents the inner part of the l)OX from heating. At both ends 

 there are iirojectino; pieces, which widen so as to cover the 

 microscope a[)paratus. and shut out the side lii^ht from the 

 olijects observed and the sun from the ends of the bar. They 

 move on hinges, in order to turn them back on the sid<>s of 

 the box and to prevent them from injury. This airange- 

 ment is seen at IMate IV. 



At the top of the side of the hox, near the end where the 

 screw motions are, a brass support c Is adapted, having a 

 te|('seoi)e exacllv equal to those on the ten inch sextants, 

 and adjustable exactly in the same maimer in its hori/onial 

 diieclion. It is of small mignifying [)ower, so that a pin. /J 

 erected at the other extremity of the Iiox. mav be seen with- 

 out parallax at the same time with a distiint signal rlirecling 

 the measurement. It has a verti'al wiic in tin* focus to ef- 

 fect the adjustment of the box inde|)endeiit of the pin. It 

 may not be amiss to place two such telescopic arrangements 



