OF THE UNITED STATES. XiSU 



tliroiiG;li the drum, and is fastened to it l)v the three circular 

 plates tbrniino; the toj), the bottom, and the middle plate ol 

 the drum /, Plate V. fig. 3. 



This drum is nine inches in diameter and five inches and 

 a half ill heip;ht. From tlie side of it project three horizon- 

 tal amis /, /, /, at l:iO degrees from each other, which are 

 hollow truricated cones similar to the radii of the horizontal 

 circle, and of the same diameter near the drum, but much 

 more tapering. Each of them bears at the end a strong 

 piece (/(/. with a circular vertical hole to receive the com- 

 pound micrometer microsco])es by which the divisions are 

 read off. 



1 hcse microscopes are six inches long, and with a mag- 

 nifying power of about fourteen times. They are marked 

 A, B, C, in the direction of the numbers of the gradation. 

 The degrees are only read by the microscope marked A, 

 and are there indicated i)y the light index m. which projects 

 from below the arm of the microscope, and folds back upon 

 it when not in use. When in use, it is made visible by the 

 microscope, when laid out so as to point close to the divi- 

 sion, without however touching the arch, in order not to 

 scratch it. The field of the microsco|)es embraces about 

 4,.T minutes of the division, so that there is never any ditTi- 

 culty in reading the smaller parts. The thirty minute marks 

 being extend'.-d entirely across, like those of degrees, and the 

 degrees distinguished by a strong dot in the protracted |)art 

 of the line, always one or the other appearing in the field. 

 The micrometer heads nn read in the inverted direction, on 

 account of the inversion of the image i)y the microscopes, so 

 tliat while the degrees go from right to left, the reading in 

 the microscopes presents itself directly, or from left to 

 right. 



These microscopes being well known by various descrip- 

 tions, it would not lie proper to enter here into further de- 

 tails resprctim:!; them. They can l)e adjusted to 120 de- 

 grees by a small horizontal motion, which they admit in the 

 pieces q, directed and fastened by three screws on the side 



