461 



cylinder is 2 T 6 n inches in length, and half an inch in diameter. It is 

 polished to the highest lustre. Within the glass tube is a Thermometer, 

 the mercurial reservoir of which is cylindrical, and about the same 

 length as the silver cylinder. When the Thermometer is in its place, 

 and ether poured in, the mercurial reservoir — which for the future I 

 shall call its bulb — is immersed. The graduated scale is fixed outside 

 the glass tube, which contains the stem of the Thermometer, and the 

 indications of the mercurial thread can easily be read through the 

 tube with precision. The scale is graduated to every half-degree 

 between 15° and 110°, and quarter- degrees or less can be easily read off. 

 The Thermometer is cemented at top to a brass cap, carrying an ad- 

 justing screw, which permits the precise adaptation of the mercurial 

 thread within to the freezing point marked on the scale without — an 

 adaptation the more necessary, as the Thermometer sometimes requires 

 to be taken out and cleansed from an oily matter deposited by the 

 ether during its evaporation. 



The brass socket, which sustains the glass tube and included Ther- 

 mometer, has an air passage drilled through its axis, and continued 

 through the horizontal piece down through the axis of the pillar to 

 the edge of the circular brass foot, where it ends in a stop-cock ; to 

 this may occasionally be attached a flexible metal tube, * of the 

 smallest bore, six or eight feet long, to the other end of which is 

 occasionally connected a small exhausting syringe ; the piston should 

 work rather freely, as much exhaustion is not necessary ; the valve 

 should be of sheep's bladder. 



The effect of this arrangement is, that by working the piston the 

 glass tube containing the thermometer is more or less exhausted ; the 

 ether bubbles up, and evaporates rapidly or slowly as required ; the 

 silver cylinder, after a while, suddenly becomes dull with condensed 

 atmospheric aqueous vapour, and the Thermometer within indicates the 

 temperature at which the first dulness had taken place. This is the 

 dew point, under certain restrictions, to be described hereafter. 



In the bottom of the silver cylinder is soldered a very slender 

 recurved silver tube, the bore of which is not quite one-twentieth of an 

 inch ; it communicates with the interior of the silver cylinder, not only 

 at the bottom, but by three minute equidistant holes, where it passes 

 up, and is soldered to the side of the ferule ; it continues along the back 

 of the therm ometric scale, and terminates at the top in a slightly tapering, 

 almost cylindrical funnel. Through this the ether is introduced, which 

 then runs down into the cylinder beneath, fills it, and rises about half 

 an inch into the glass tube, on which the cylinder is cemented. The 

 above-mentioned cylindrical funnel serves another very important pur- 

 pose that will be described hereafter. 



* Indian rubber, although more convenient, would be soon destroyed. 



