EELATIOX OF VAPOR AND AIR. 177 



therefore, in this form, inconsistent "with Dalton's principle ; but it is 

 not difficult to modify the expression so as to retain the essential part 

 of the explanation. 



Dew. The principle of a " constituent temperature" of steam, and 

 the explanation of the " dew-point," were known, as we have said 

 (chap. iii. sect. 3,) to the meteorologists of the last century ; but we 

 perceive how iucomplete their knowledge was, by the very gradual 

 manner in which the consequences of this principle were traced out. 

 We have already noticed, as one of the books which most drew atten- 

 tion to the true doctrine, in this country at least, Dr. "Wells's Essay on 

 Deio, published in 1814. In this work the author gives an account of 

 the progress of his opinions ; " "I was led," he says, " in the autumn 

 of 1784, by the event of a rude experiment, to think it probable that 

 the formation of dew is attended with the production of cold." This 

 was confirmed by the experiments of others. But some years after. 

 " upon considering the subject more closely, I began to suspect that 

 Mr. Wilson, Mr. Six, and myself, had all committed an error in regard- 

 ing the cold which accompanies the dew, as an effect of the formation 

 of the dew." He now considered it rather as the cause : and soon 

 found that he was able to account for the circumstances of this forma- 

 tion, many of them curious and paradoxical, by supposing the bodies 

 on which dew is deposited, to be cooled down, by radiation into the 

 clear night-sky, to the proper temperature. The same principle will 

 obviously explain the formation of mists over streams and lakes when 

 the air is cooler than the water ; which was put forward by Davy, 

 even in 1819, as a new doctrine, or at least not familiar. 



Hygrometers. According as air has more or less of vapor in com- 

 parison with that which its temperature and pressure enable it to con- 

 tain, it is more or less humid ; and an instrument which measures the 

 degrees of such a gradation is a hygrometer. The hygrometers which 

 were at first invented, were those which measured the moisture by its 

 effect in producing expansion or contraction in certain organic sub- 

 stances ; thus De Saussure devised a hair-hygrometer, De Luc a 

 whalebone-hygrometer, and Dalton used a piece of whipcord. All 

 these contrivances were variable in the amount of their indications 

 under the same circumstances ; and, moreover, it was not easy to 

 know the physical meaning of the degree indicated. The dew-point, 

 or constituent temperature of the vapor which exists in the air, is, on 



52 Essay on Dew, p. 1. 

 VOL. II. 12 



