458 Morley and Brush — Measurement of Small Pressures. 



uniformity of temperature, we made this connecting tube two 

 centimeters in diameter. With such freedom of communica- 

 tion, anywhere in a city, there is a continual slight oscillation 

 of the mercury between the two arms of the gauge. Accord- 

 ingly, in the second apparatus constructed, we damped the 

 oscillation by putting a thin metallic diaphragm in the 

 connecting tube. This may consist of a platinum cone tied in 

 place with a wire and a wedge of glass, as suggested by figure 3. 



An important matter in the construction of such a gauge 

 needs mention. It is but through a small part of the surface 

 of a glass tube that good definition of the points can be had 

 with the microscope. One must select a number of such 

 places in his stock of glass tubes, and mark the best of such 

 favorable places. In the completed apparatus, the selected 

 areas are brought to the predetermined position. 



The mounting of the apparatus has to be about as stable as 

 that of an astronomical instrument. It must be so designed 

 that distortion shall not be caused by changes of temperature. 

 In our gauges, a massive cast-iron standard rests on a isolated 

 stone pier. On this, moving in trunnions like those of a 

 transit instrument, is carried the plate to which gauge, mirrors, 

 and microscope are fixed. With such stability, and with 

 many precautions, we have been able to measure small 

 pressures with a mean error not much greater than the ten- 

 thousandth part of a millimeter. 



It may be noted that in the first form of gauge it is best to 

 use points of platinum, for glass points can not well be given 

 the form permitting most convenient manipulation, that of a 

 right cone of small angle. The adhesion of mercury and glass 

 also causes much loss of time. In the second form, where the 

 points must end in minute hemispheres, and where actual 

 contact of mercury and the point is not required, glass points 

 can be used, but the ease with which platinum points can be 

 given a proper outline in the lathe speaks in their favor. 



Note — Since this paper was written, we have learned that Lord Eayleigh 

 has used with success a gauge much resembling the first form used by us ; 

 he determined the inclination given the gauge in making the points tangent 

 to the mercury, by observing with a reading telescope the inclination given 

 to a mirror carried with the gauge. 



Adelbert College, Cleveland, Ohio. 



