400 RECORD OF SCIENCE FOR 1887 AND 1888. 



line of saturation has not been attained ; that is to say, when the changes 

 have occurred entirely within the dry stage. 



(2) Or, on the other hand, and which generally happens, the water 

 that is formed as the indicator passes below /; is separated from the rest 

 of the mass by falling to or towards the earth, and be is a pseudo-adi- 

 abatic; in this case the change is irreversible, and as the air descends 

 along the opposite slope of the mountain the indicator follows a curve 

 cd other than be, or the adiabatic of the dry stage. 



We see by the consideration of Fig. 5, that the initial temperature 

 will be recovered at a pressure lower than the initial pressure ; that the 

 temperatures that are attained become higher and higher, and the ex- 

 cess over the former becomes greater in proportion as the air descends 

 and is compressed. 



Moreover, the absolute quantity of the aqueous vapor has become 

 smaller, therefore the real line parallel to the plane pov, and of which 

 cd is the projection, is now much nearer to this plane than was the orig- 

 inal line ab. 



Thus we have cold and moist air on one side of the mountain becom- 

 ing warm and dry air on the other. Thus we derive all the character- 

 istic properties of the Foehn, and explain without difficulty how it is 

 that these properties do not pertain to descending winds that have not 

 first surmounted a summit, for example, to the winds that simply de- 

 scend along the slope of a plateau. 



(B) Interchange of air between a cyclone and anti cyclone, in the sum- 

 mer. — The changes of condition of air within a cyclone and anti-cyclone 

 are analogous to those of air that has surmounted a mountain ridge. 

 Eain within the area of a cyclone, dry weather in a clear sky within an 

 anti-cyclone. But whereas, in the case of the Foehn, these changes are 

 experienced within an area of small diameter, so that one can neglect 

 external thermal actions ; the passage from the cyclonic state to the 

 anti-cyclonic is, on the contrary, effected within a space so extended 

 that it is necessary to take account of these actions. 



In summer the addition of solar heat is the prevailing power; in 

 winter it is the reverse ; the day-time and the night timehave influences 

 very nearly like those of summer and winter. In all cases the curves 

 representing the condition are no longer adiabatic. 



Let us suppose that the air passes from the cyclonic to the anti-cy- 

 clonic condition in the summer time. Starting from an initial condition 

 a in the cyclone, the temperature diminishes by expansion and the indi- 

 cator of the condition of this mass of air starts to describe the adia- 

 batic ab\ but the diminution of temperature is retarded by the addition 

 of external heat and the air expands, as shown by the movement of the 

 indicator along the curve ab, which is less inclined than the adiabatic 

 ab^. It follows that the line of saturation is attained later than it other- 

 wise would be, and this corresponds to a higher elevation above the 

 surface of the earth of the mass of air under consideration. (See Fig. 6.) 



