OF THE UNITED STATiis. ^79 



The pieces DD, wliich arc thus tlie iinniediiUe bearers of 

 the bar, are fastened upon a wooden quadrangular l)ar H 

 of two inches square and al)out twenty-six I'eet Ion;:;, reccd- 

 ing about seven inches from each end of the iron l)ar, Plate 

 III. tig. -1-. and Plate IV. 'I'o prevent, as much as possible, this 

 wooden bar from warping in (he air, it was formed of strips 

 of wild cherry, glued together so as to break joints, 'I'o 

 move this bar H in the direction of its length, and thereby 

 also the iron bars, there is a brass frame K, Plate III. fig. 4 

 t^ o, and I'lale IV. with a screw anangement adapted to one 

 end of it. by three screws LL going through the wooden 

 bar and the two brass plates which embrace it on both sides. 

 Two strong perpendicular pieces iMM, Plate III. iig. 4, of 

 which one forms the butt end of the bar H, and the other 

 is held parallel to it at the distance of eight inches, form the 

 socket of the axis of the steel screw Q, which is about 0,6 

 inches in diameter, and which is hid in the brass framing K. 

 It is turned by the mdled liead screw I, seen at the end ot 

 this frame, perpendicularly under the bar. 



The piece P, which has the mother screw corresponding 

 to Q, is fixed solid to the O, which lies flat in the bottom of 

 the box (o which it is fastened f)y four strong screws passing 

 through the brass and the bottom board, and made fast by 

 mother screws from the lower side of the board. 



I he piece P, having the mother screw corresponding to 

 Q, has two friction rollers KU on its side, Plate III. fig. 5, 

 which press against the upper plate N of the brass frame 

 K, to prevent any rotatory vacillation by the friction of tlie 

 screw. 



By the screw Q, the measuring f)ars are evidently moved 

 in the box in the direction of their length. This motion is 

 fully eased \)y eleven rollers SS of about half an irjch in 

 diameter, passing under the whole l)rcadth of the bar H. 

 All lateral motion is prevented l)y steel springs T, pressing 

 moderately against it from both sides, and which are fast to 

 tfie uprights holding the rollers, so that the contact of the 



s 



