486 ON IMPRESSIONS OF COLD 



agreement between theory and observation is most satisfac- 

 tory. 



3. Pendant. — The sethrioscope might be reduced to a small- 

 er and more compact form, by conjoining with it a pendant 

 differential thermometer. Neither of the glass balls in this 

 case requires to be gilt. But of the Pendant iEthrioscopes 

 there are two varieties. In theirs/, the balls of the pyroscope 

 are enclosed by hemispherical brass cups. (See Fig. 6. Plate 

 XL) These, however, may be a little deeper, forming seg- 

 ments about 10 or 15 degrees more than the hemisphere. 

 The pyroscopic balls, with two-fifths of the diameter of the 

 cups, should be placed in the middle, between the centre and 

 the bottom. In the second variety, the lower ball of the pyro- 

 scope is encased by a hollow sphere of brass, composed of two 

 pieces which screw together, and the upper ball occupies the 

 focus of the cup, which needs scarcely be more than two inches 

 wide. (See Fig. 4.) This variety of the instrument is more 

 portable than any of the rest, and equally accurate ; but owing 

 to the brass casing, it is, under a change of temperature, rather 

 slower in its action. The construction is rendered still more 

 commodious, by having the stem of the differential thermome- 

 ter inserted through a projecting circular piece, somewhat lar- 

 ger than the upper ball, which screws into a perforation at the 

 bottom of the cup, and then forms part of the same reflecting 

 cavity. This arrangement allows not only the standard of the 

 instrument to be occasionally detached, but also the cup itself, 



a circumstance at once conducive to the safety of carriage, 



and to the preservation of the metallic lustre and polish. 



On replacing the metallic lid, the effect is entirely extin- 

 guished, and the fluid in the stem of the differential thermo- 

 meter immediately sinks to zero. A cover of pasteboard has 

 at first precisely the same influence ; but after it has itself be- 

 come chilled by this exposure, it produces a small secondary ac- 

 tion 



