248 Mr C. Martins on the Nature and Origin of Dry Fogs. 



subject of study as new as it is interesting. But to enter 

 upon it with the well-founded hope of resolving the question, 

 we beg of observers to employ M. Regnault's aspirator 

 along with hygrometrical instruments. This instrument 

 enables us to weigh the quantity of aqueous vapour contained 

 in a given volume of air, and it thence follows that the re- 

 sults are free from all the causes of error which may attach 

 to Saussure's hygrometer, August's psychrometer, that of 

 Daniell, and even M. Regnault's condenser. In a thick fog 

 the eye can scarcely notice the exact moment when the vapour 

 is deposited in small drops on the silver capsule of the con- 

 denser, or of Daniell's hygrometer, and consequently there 

 is always uncertainty as to the degree of the thermometer at 

 which the dew is deposited. Travelling meteorologists will 

 be much to blame if they neglect the hygrometrical study of 

 fogs, carried on by means of these portable instruments. An 

 approximating result is always preferable to absolute igno- 

 rance. Our numerical data and physical experiments are 

 exact, compared with those of our predecessors. But our 

 successors, provided with more delicate apparatus, and more 

 extensive knowledge, will find precisely the same faults with 

 us that we sometimes allege against those who have gone 

 before us in the same path. Absolute truth is a phantom 

 which man continually approaches, with the certainty of 

 never reaching it. 



