Daniell on a new Hygromeler. 30.5 



of September, it will be seen that a very remarkable rise in 

 the temperature of the vapour preceded by many hours the 

 change of the wind, on the 4th, from N.W. to S.W. A 

 gradual fall on the 20th of October preceded the change of the 

 wind from S.W, to N.W. on the 21st. The southern and 

 western sea-breezes bear with them a very different proportion 

 of vapour to that of the northern and eastern land breezes. 

 The reason why the aqueous particles may appear to precede 

 the wind, I shall endeavour to discuss in a future paper, to- 

 gether with some other points of theory, on which I think the 

 present detail of facts and experiments may throw some 

 light. In cases of mist, fog, and cloud, the instrument 

 will sometimes exhibit a different kind of action. If it be 

 brought from an atmosphere of a higher temperature into 

 one of a lower degree, in which condensed aqueous particles 

 are floating, the mist will begin to form upon the ball at a 

 temperature several degrees higher than that of the air. The 

 difference, I believe, is proportionate to the density of the cloud 

 or mist ; but I speak with diffidence upon this point, as I have 

 not had sufficient opportunities of verifying it by experiment. 

 I have sometimes thought, that I have perceived a difference in 

 this respect, in different modifications of the cloud, but this 

 must be referred to future more extended observations. This 

 action upon floating water does not at all interfere with it as 

 measuring the force and quantity of vapour, for in all such 

 cases the full saturation of the atmospheric temperature must 

 have place, and consequently the temperature of the vapour 

 must be coincident with that of the air. 



But although the hygrometer which I am now describing 

 excels all oth«rs, I believe, in sensibility, and the accuracy 

 with which it marks the comparative degrees of moisture and 

 dryness in the atmosphere, and, by exhibiting them in degrees 

 of the thermometer, refers them to a known standard of com- 

 parison, and speaks in a language which every body under- 

 stands, yet it is not upon this alone that I venture to found its 

 claims of superiority. Its great merit (M^nsists in indicating 

 with ea»e and precision the positive weight of a(iueoiis gas 



