OP THE UNITED STATES. 259 



For the actual comparison of tlic metres and these hars. 

 it was necessary to place tlicin on tlie work bench, on wliich 

 tile above described comparator and English brass scale were, 

 at the exact height, which would bring their upper surface, 

 without parallax, to the foci of the microscopes, which were 

 of course adjusted for the divisions of the scale. The influ- 

 ence of unequally supporting the standard bars, by merely 

 laying someiliing uuvier them at diftcrent places, being great. 

 I caused pine rules to be made, of sufficient breadtli and 

 length, and of the exact thickness required for each stand- 

 ard, L'pon these each standard was laid, together with its 

 proper butting pieces, when under comparison. 



As it is wholly inadmissible to take the edge of a bar as 

 an object under the microscope for the purpose of compa- 

 rison, because it never gives a good image, the shorter pieces 

 cut from the bars, from two to four inches in lengtli, were 

 filed on the tool, in the manner of the butting pieces above 

 described, brought to the exact thickness of the standard 

 with which they were intended to be used, and then the butt- 

 ing faces of them rubbed against each other with emery and 

 oil, upon the iron plate of the tiling tool, constantly inverting 

 their positions, until such a perfect contact was obtained, 

 that the line formed by it was not so thick as one of the di- 

 visions of the scale. 



These pieces were always laid against the ends of the 

 standard under comparison, so tliat the junction appeared 

 like a line drawn upon the standard, with which tiie cross 

 wires of the micrometer were made to intersect. The mi- 

 croscopes were furnisiicd witii reflectors, formed of white 

 paper placed in a position inclining forwards, between the 

 microscopes and their supports. By these the light was 

 reflected upon the scale, or tlie standards, in the direction 

 of the division lines, as required for accurate reading. 



To prevent the heat of my bofly from having any influence 

 on the scale and apparatus, a large sheet of paper was nailed 

 to the work bench near the microsco|)es, and I worked with 

 gloves on. 



