OF AETS AND SCIENCES. 31 



f, gauges C, B, A (reversing order on ends of column also), tempera- 

 ture #, barometer, and finally the readings of auxiliary thermometers 

 for temperatures of gauges, etc. 



In order to eliminate as far as possible any uncertainty in setting the 

 cross-hairs of the telescope upon the true top of the mercury meniscus, 

 a difficulty requiring special precaution when readings to 0.01 mm. are 

 attempted, the device was adopted of placing about the tube a clasp 

 with a dull black internal surface. This clasp was placed upon the 

 tube above the mercury column, and was pushed down the tube until 

 the plane of its lower edge was nearly tangent to the mercury menis- 

 cus, which then appeared black against a bright background otherwise 

 secured for it. Another convenient method is to make the meniscus 

 bright against a dark background, which is accomplished readily by 

 placing behind the tube a piece of paper separated along a horizontal 

 line into two portions, the upper of which is white, the lower black. 

 When a reading is to be taken, this is adjusted at such a height that 

 the black shall be just visible above the top of the meniscus. In both 

 of these methods it is essential that care be taken that the adjusted 

 black edge be not more than about one fifth of a millimeter above the 

 top of the meniscus, as otherwise error may be introduced through 

 mistaking the reflection of the line by the meniscus for the actual top 

 of the meniscus. 



Errors in observation of the height of the barometer affect p^, p^ 

 and 2\ by equal amounts in any given experiment. The effect of such 

 errors will therefore be small, and its magnitude, given in the last 

 column of the preceding table, may be demonstrated in the following 

 manner. In the expression 



may be inserted p^ = I{-\- c, p., = II-\-h, p^=: ff-\-a, where // 

 is the barometric reading and c, b, and a are the gauge readings, at 

 entrance, middle, and exit of the apparatus. Thus, 



r^ 'b — a / a — c \ 



y = ^- c-^ir^ii + Q^TH^) 



• i?l- — _ TT \ (i\ "~^ 2 (fl — c) 



•• dE~ "^ '"^y^ ' h — c' {:2H-\-b-\^cy 



b — a 2 (a — c) A • f (0 ^ 



