416 Mr. C. F. Brush on the Measurement 



McLeod gauge which it is the purpose of this paper to discuss. 

 It was deemed necessary to provide an apparatus capable of 

 easy use, and giving results having a probable error not 

 greater than a fiftieth part of a millionth of atmospheric 

 pressure. My hopes of attaining this high ideal were far 

 more than realized, as the sequel will show. Instead of having 

 to wait days or weeks, in the course of my investigations, for 

 the evolution of a measurable quantity of gas as had been 

 expected, the progress of evolution could be noted from hour 

 to hour; and the increase in the rate of evolution due to a 

 rise of one degree in the temperature of the laboratory was 

 unmistakable. 



The diagram herewith shows the essential parts of my 

 apparatus. The bulb A, of the gauge, is made conical in its 

 upper part to avoid adhesion of gas bubbles when the mercury 

 rises. This bulb holds about eleven pounds of mercury. 

 B and C are the gauge-head and comparison tube respectively. 

 They are nearly 20 millimetres inside diameter, and are made 

 from contiguous parts of the same carefully selected tube. 

 D is the usual air-trap, and E is a long glass tube, with flexible 

 pure rubber connexions to the lower end of the gauge stem 

 and the mercury cistern F. The latter is mounted on a 

 carriage G-, which moves vertically on fixed guides. The 

 height of the carriage is adjustable, at the upper end of its 

 range of motion, by means of the screw H, thumb-nut I, and 

 forked support K. The screw is pivoted to the carriage, so 

 that it may swing out of the fork when the carriage is lowered. 

 L is a pinch-cock with screw, for regulating the flow of 

 mercury, or stopping it altogether, while pumping out the 

 trap D. N is a bulb containing phosphorus pentoxide, to 

 keep the interior of the gauge and other parts of the apparatus 

 perfectly dry. P is a very elaborate cathetometer for observing 

 the mercury columns in B and C. This beautiful instrument 

 has a revolving column with vertical scale, and vernier with 

 microscope, reading to hundredths of a millimetre. The eye- 

 piece micrometer reads directly to hundredths of a millimetre, 

 and the divisions on the revolving head of the screw are so 

 open, that tenths of divisions are easily and certainly estimated 

 by an experienced eye ; thus permitting the micrometer to be 

 read directly to thousandths of a millimetre. Of course the 

 cathetometer is perfectly located, not as shown, but with the 

 objective of its telescope equally distant from the axes of the 

 tubes B and C when it is alternately directed to them ; and 

 at such a distance that its micrometer readings correspond to 

 a millimetre scale. The whole apparatus is located in a base- 

 ment room, on a stone floor, whereby vibrations are reduced 

 to a minimum. 



