40 Mr P'oggo on the Dezv-Point Hygrometer. 



perature perhaps, and have all this heat to part with before it 

 is condensed. But what is the use of this long tube, unless 

 because there is such a one in the cryophorus ? The reason 

 for being long in that instrument is obvious, but the objects 

 of both instruments are diametrically opposite. Dr Wollaston 

 wished to effect freezing in the most disadvantageous circum- 

 stances, in order to show the force of the principle, but in the 

 hygrometer the purpose manifestly is to produce cold most 

 advantageously and economically. The principle of this hy- 

 grometer is beautiful, but the execution is not very happy. 

 It is uselessly large. The only efficient parts are the two balls 

 and the included thermometer, which, though the case must 

 be larger than an octavo volume, is much too small for accu- 

 rate observation. There is a great waste of ether, which at 

 the end of the year must be no small quantity spent. There 

 is a great loss of power in cooling a large body of ether, 

 none of which, except the superficial film, is allowed to act. 

 The principle of producing the cold by distillation in vacuo 

 may be successfully given up altogether. Since the evaporation 

 from the covered ball is able to depress the thermometer in 

 the sentient ball a considerable number of degrees, a much 

 more intense effect might be produced were the same quantity 

 of ether suffered to evaporate from the surface of the ether at 

 once ; or, in other words, the same effects might be produced 

 by a much smaller quantity lost. ' To accomplish this, all that 

 is necessary is to expose the surface of the ether at once to the 

 action of the atmosphere, to surround the bulb of the thermo- 

 meter by a cup, into which pour ether, and, having observed 

 the dew-point on the surface of that, return the ether to the 

 phial. Here every drop of ether evaporated would be em- 

 ployed in producing the requisite degree of cold. The flat 

 bottomed-cup I propose should have a lid to prevent or modi- 

 fy evaporation when the dew-point comes out speedily. The 

 cup also shps off, and leaves the thermometer fit for ascertain- 

 ing the temperature of the air, and entirely supersedes the use 

 of two thermometers in making a hygrometrical observation." 

 On these hints, the hygrometer which I formerly described 

 was constructed by Mr Coldstream, and used by us in our 

 observations made at Leith, and published in the EdinhurgU 



