344 LIEUT.-COLONEL SYKES’S DISCUSSION OF METEOROLOGICAL 
to remark particularly the localities of dew at Poona and in its neighbourhood. In 
September and October I found that when there was not a trace of dew in the can- 
tonment, there would be a deposition on the fields of standing grain half a mile 
distant, and when there was not any dew either in the cantonment or in the fields, it 
would yet be found on the banks of running rivulets and on the banks of the Mota 
Mola River ; but with respect to the rivulets, fifteen or twenty feet from the water 
were the limits of the deposition!’’ 1 gave numerous other instances of the local deposi- 
tion of dew proximate to irrigated lands, or in the neighbourhood of water, indicating 
the suspension of vapours over the localities, in complete analogy with what occurs 
to the wet bulb thermometer when the air is calm. That agitation of the air is 
necessary to disperse the vapour surrounding a wet bulb, has been noticed by British 
chemists. Brand says (page 111, last edition), “ It is now established that the pres- 
sure of air is really an obstacle to evaporation, and that a current is useful, not by 
supplying new quantities of air, but by removing the vapour according as it is formed 
and leaving fresh spaces into which the vapours may expand.” He elsewhere says 
(page 82), “Evaporation is proportional to the surface exposed ; it is also accelerated 
by agitating the superincumbent air, as in the case of a brisk wind, or by artificial 
means. When the air is tranquil the vapour rests upon the surface of the water, and 
it is the pressure of its own vapour on the surface of a liquid, and not that of the gaseous 
atmosphere which stops the process.” M. Regnault has demonstrated the truth of 
this in an elaborate manner. Accounting for the different results of observations in 
a closed and open chamber, he says the wet bulb was not sufficiently depressed in the 
closed chamber. “Cette circonstance tient evidemment a ce que fair se trouve 
beaucoup moins agite qu’a rexterieur*.” After experimenting in a room with two 
windows open, he adds, as before stated, “Ces experiences demontrent de la maniere 
la plus evidente que la formule ne pent pas rester la meme pour divers etats d’agitation 
de Fair.” (P. 220.) The vapour therefore resting upon the wet bulb is a source of 
error, but the removal of it leads to one much more grave. M. Regnault, in refer- 
ence to M. August’s formula, says (p. 207), “ La formule ne tient aucun compte de 
la vitesse du courant d’air ; d’apres cette formule, la difference de temperature devrait 
etre la meme, quelle que soit cette vitesse. Ce resultat parait impossible a priori. 
J’ai cherche a determiner par des experiences directes, Finfluence de cette vitesse et a 
reconnaitre si, a partir d’une certaine valeur de la vitesse, les differences de tempera- 
ture des thermometres sec et mouille deviendraient independentes de la vitesse 
absolue du courant d’air, consequence a laquelle on se trouve naturellernent conduit 
par le raisonnement que M. August applique au calcul de la formule du psychro- 
inetre.” M. Regnault then describes his apparatus and mode of making his experi- 
ments. He gives two series of experiments ; in the second experiment the air being 
made to blow upon the wet bulb with a greater velocity than in the first. It will be 
sufficient to give the first and sixth figures of each series. 
* Annales de Chimie, tom. xv. p. 219. 
