128 Daniell on a new Hi/grometer. 



opinion. It is replete with acknowledgments of the obvious 

 defects of instruments constructed upon this principle,— defects 

 which it was impossible that a mind like his could overlook, 

 and it proves fully that his sagacity and genius were tasked to 

 the utmost to diminish sources of uncertainty, which it was 

 out of his power wholly to remove. Any person who had not 

 seen the minute instructions given by this able philosopher 

 for the construction of his hygrometer would be surprised at 

 the nicety required in its adjustment. The mere preparation 

 of the hair is a process of great delicacy and uncertainty. It is 

 previously exposed to an alcaline lixivium, upon the due 

 strength and regulated application of which, depends its most 

 valuable properties; and hairs which have been unequally 

 exposed to this action are no longer fit for comparisons with 

 ■one another. " Les cheveux n'ont une marche parall^le, que 

 quand ils sont egalement lessives." So that it would be im- 

 possible for an artist in London, with the most scrupulous 

 attention to the directions contained in this Essay, to con- 

 struct an instrument which should range with one made in 

 Geneva, unless he had the means of actual comparison. 

 But after all the care which the ingenuity of such a philosopher 

 could devise, (and none but such a philosopher could be com- 

 petent to take such precautions,) thus guardedly and candidly 

 does the inventor speak of the best instruments : " Quant a la 

 comparabilite des hygrometres construits avec cette substance, 

 je puis dire que deux ou plusieurs de ces instrumens faits avec 

 des cheveux semblablement prepares, gradues sur les m6mes 

 principes, et exposes ensuite aux m^mes variations d'humi- 

 dite et de secheresse ont des marches que Ton pent nommer 

 paranoics. Je ne dirai cependant pas qu'ils indiquent toujours 

 tons le m^me degre, mais que leurs ecarts vont rarement au- 

 de\k de deux degres. Si apres que deux hygrometres auront 

 sejournes pendant long-tems dans un air tres sec, par exemple, 

 au quaranti^me degre de ma division, on en porte un dans un 

 air encore plus sec qui le fasse venir je suppose k trente, et 

 que pendant ce tems-1^, Tautre ait ete porte dans un air un 

 peu moins sec, par exemple, a cinquante degres; qu'ensuite on 



