Daniell on a new Hygrometer, 127 



highly interesting to a naturalist, who can transport them from 

 the temperate to the torrid zone, from the northern to the 

 southern hemisphere, from the low regions of the air which rest 

 on the sea, to the snowy tops of the Cordilleras." 



" I have never been able to reduce the hair on whalebone to 

 the degree of extreme siccity for want of a portable apparatus, 

 which I regret not having made before my departure. I advise 

 travellers to provide themselves with a narrow jar containing 

 caustic potash, quick -lime, or muriate of lime, and closed 

 with a screw by a plate, on which the hygrometer may be 

 fixed. This small apparatus would be of easy conveyance, if 

 care were taken to keep it always in a perpendicular position. 

 As under the tropics, Saussure's hygrometer generally keeps 

 above 83, a frequent verification of the single point of humi- 

 dity is most commonly sufficient to give confidence to the 

 observer. Besides, in order to know on which side the error 

 lies, we should remember that old hygrometers, if not cor- 

 rected, have a tendency to indicate too great dryness." 



Mr. Leslie, in his Essay upon the Relations of Air to Heat 

 and Moisture^ makes the following remarks upon the same 

 subject. *' But these substances, (viz., hygroscopic sub- 

 stances,) especially the harder kinds of them, unless they be 

 extremely thin, receive their impressions very slowly, and 

 hence they cannot mark with any precision, the fleeting and 

 momentary state of the ambient medium." " The expansion 

 of the thin cross sections of box or other hard wood, the 

 elongation of the human hair, or of a slice of whalebone, and 

 the untwisting of the wild oat, of catgut, of a cord or linen 

 thread, and of a species of grass brought from India, have, at 

 different times, been used with various success. But the in- 

 struments so formed are either extremely dull in their motions, 

 or if they acquire greater sensibility from the attenuation of 

 their substance, they are likewise rendered the more subject 

 to accidental injury and derangement, and all of them appear 

 to lose in time insensibly their tone and proper action." 



But it is to the Essay of Mr. De Saussure himself, that I 

 appeal with the most confidence for the confirmation of this 



