1862.] 55 



not exceed that of the atmosphere, consisting of a glass globe (a) 

 3 inches in diameter, and stem 35 inches long; the capacity was 

 known to a point (6), where a piece of platinum wire was twisted to 

 mark accurately the point at which the mercury column in the stem 

 was to be brought to maintain a constant volume in the globe. 



A l|-inch tube (e?), filled with* mercury, rested upon the frame 

 (e e). The weight of the tube and frame was counterbalanced by 

 weights (w w). By such an adjustment the tube (d) could be regu- 

 lated with facility, preserving the upper level of the mercury column 

 at one uniform height. A cathetometer (g g}, with vernier (A), to 

 read the lower and variable level of the mercury column, was intro- 

 duced. To heat the globe, the oil-bath (k) was used, fitted to the 

 tube (t) by a stuffing-box (s) ; the oil-bath is itself immersed in a 

 mercury bath (Q, surrounded by a coil of jets of gas (m m). 



The globe, filled with dry and warm mercury, the air-bubbles 

 being extracted by means of an air-pump, was inverted to form a 

 Torricellian vacuum. A small glass globule of water was then in- 

 serted, the platinum wire fixed in its place, and an india-rubber cap 

 fitted to the extremity of the stem. Being transferred to its place, 

 and the india-rubber cap replaced by an open glass cistern, so that 

 the glass (d) could be elevated to its position, the jets were lighted, 

 and the temperature elevated to 300. 



From this point the levels of the column were read off at intervals 

 of 50 until the temperature of saturation was reached. The levels 

 were taken in a series of descending temperatures, to avoid the in- 

 fluence of steam boiling out of the mercury as the temperature rose, 

 and to eliminate the effect of the cohesion of the glass on the water, 

 as explained in our previous paper on the density of steam. 



Twelve cubic inches of mercury were measured into the globe, and 

 a file-mark made on the stem, below which, at a distance of 14*45 

 inches, another file-mark was made, affording a fixed point for ascer- 

 taining the correspondence of the upper file-mark with the readings 

 on the cathetometer. 



Let a be the reading on the fixed rod of the level of the column, 

 b the reading of the lower file-mark on the globe-stem; then 6 a 

 = the height of the column of mercury on the globe-stem. 



To correct for temperature, 1\ inches of mercury, enclosed by the 

 oil-bath and its stuffing-box, were corrected for the temperature of 



