Dr. Lyon Play fair on the Milk of the Cow. 281 



frequently follows the change of the wind from north-east to 

 south-east), and the latent heat emitted during the condensa- 

 tion may raise the temperature of the air so as to diminish its 

 density sufficiently to enable the diffusive force of succeeding 

 portions of vapour to carry down some of the air of the upper 

 current, which, by immediately checking the air flowing in the 

 opposite direction by the force of its momentum, will be fol- 

 lowed by another quantity of air to supply the want occa- 

 sioned by the check; and thus when the difference in the 

 quantity of vapour in the opposite currents is sufficient to 

 cause a tolerably abundant flow of the vapour of the upper one 

 into the strata of the lower, the upper current may, by the 

 above process, gradually work its way to the surface of the 

 earth. 



Although this hypothesis is offered as little more than a 

 conjecture, observation has afforded me some rather striking 

 instances of a change of wind from north to south immediately 

 following a sudden dryness of the atmosphere ; and as rain so 

 frequently occurs at the changing of the wind to south, it may 

 receive some support from the results of some observations, 

 inserted by Prof. Loomis in the American Journal of Science 

 for October 1841, showing the frequent occurrence of rain 

 very shortly after an unusual dryness of the air. It is not, 

 however, proposed to the exclusion of other causes coopera- 

 ting with it, as for instance an interruption of the lower cur- 

 rent by a local elevation or depression of temperature, by 

 which a deficiency of air would somewhere be produced; but 

 that it is frequently opposed in its descent by the lower cur- 

 rent, is apparently shown by the wind changing from north- 

 east to south-east, the south wind being deflected by the force 

 of that from north-east. 



* - 



XXXIV. On the Changes in Composition of the Milk of a Cow, 



according to its Exercise and Food. By Lyon Playfair, 



Ph.D., Honorary Member of the Royal Agricultural Society 



of England*. 



A S the principal object of this paper is to draw the atten- 

 -** tion of practical men to the conditions which effect a 

 change in their dairy produce, it has been written in a more 

 elementary form than would have been done, had its purpose 

 merely been to communicate facts to scientific chemists. The 

 object of the Chemical Society is to examine the applications 

 of chemistry to practice, as well as to assist in the advance- 



* Communicated by the Chemical Society ; having been read January 

 17th, 1843. 



