126 Daniell on a new Hygrometer. 



competent to appreciate, thus speaks of hygrometers in general, 

 and of these two instruments in particular : 



* '* We know, by very accurate experiments, the capacities 

 of saturation of the air at different degrees of the thermometer : 

 but the relations which exist between the progressive lengthen- 

 ing of a hygroscopical body, and the quantities of vapour con- 

 tained in a given space, have not been appreciated with the 

 same degree of certainty. These considerations have induced 

 me to publish the indications of the hair and whalebone hygro- 

 meters just as they were observed, marking the degree shewn by 

 the thermometers connected with these two instruments." 



" As the fiftieth degree of the whalebone hygrometer corre- 

 sponds to the eighty-sixth degree of the hair hygrometer, I made 

 use of the first at sea and in the plains, while the second was 

 generally reserved for the dry air of the Cordilleras. The hair, 

 helow the sixty-ffth degree of Saussure's instrument, indicates, 

 by great variations, the smallest changes of dryness, and has 

 besides the advantage of putting itself more rapidly into a state 

 of equilibrium with the ambient air. De Luc's hygrometer acts, 

 on the contrary, with extreme slowness ; and, on the summit of 

 mountains, as I have frequently experienced to my great regret, 

 we are often uncertain whether we have not ceased our observa- 

 tions before the instrument has ceased its movement. On the 

 other hand, this hygrometer, furnished with a spring, has the 

 advantages of being strong, marking With, great exactness in very 

 moist air the least increment of the quantity of vapour in solu- 

 tion, and acting in all positions ; while Saussure's hygrometer 

 must be suspended, and is often deranged by the wind, which 

 raises the counterpoise of the index. I have thought it might 

 prove useful to travellers to mentign in this place the results 

 of an experience of several years." *' Notwithstanding the doubts 

 which have been raised in these latter times, respecting the accuracy 

 with which hair or whalebone hygrometers indicate the quantity of 

 vapours mingled in the atmospheric air, it must be admitted that, 

 even in the present state of our knowledge, these instruments are 



* De Humboldt's ^ Travels, translated by Helen Maria Williams. Vol. II. 

 p.8i,etseq. 



