346 Mr. W. S. Jevons on the Fornix of Clouds. 



head, and then sinks on every side beneath the level of precipi- 

 tation, all watery particles then evaporating. 



14. The general form and internal motions of the cumulus 

 may, indeed, be illustrated with some approach to accuracy by a 

 simple experiment in the section-glass ; by producing, in short, 

 a fountain-like jet of liquid, of which the gravity is reduced to 

 an exceedingly small amount relatively to the surrounding liquid. 



Experiment 1. Fill the section-glass with water containing 

 only a trace of common salt (1 part in 5000), and when it has 

 come to rest, introduce from the centre of the lower side a slow 

 vertical current of distilled water, of the same (ordinary) tempe- 

 rature, and containing a trace of nitrate of silver, together with 

 1 part in 5000 of its weight of sugar in solution. 



The specific gravity of the injected liquid is very slightly 

 greater than that of the remainder ; and by the effect of gravity 

 acting against momentum, is produced the regular and extremely 

 graceful fountain-like form shown in fig. 2. 



15. Either by considering the hemispherical form of the 

 natural cumulus, or the analogy which this experiment presents, 

 it is surely evident that a rising column of air, instead of being 

 perpetuated, is completely checked a short height above the plane 

 of precipitation. It is not necessary to the further progress of 

 my theory, that I should point out the whole cause of this. I 

 notice here only one point which seems to have been overlooked 

 by meteorologists, viz. that the weight of the precipitated watery 

 particles must increase the specific gravity of a cloud as a whole. 



In condensation, a certain portion of gaseous vapour, which 

 possesses only six-tenths the specific gravity of common air, is 

 reduced to liquid water, which is 815 times as dense as air. It 

 is true that this water is in the form of minute suspended par- 

 ticles or vesicles, which are not at rest but must always tend to 

 subside; yet it does not follow that their weight must not be 

 added to the weight of the air to obtain the total weight, and 

 thence the specific gravity of the whole mass. Indeed it follows 

 from the almost self-evident proposition, that the weight of a 

 body will not be affected by motions merely internal and rela- 

 tive, that the suspended particles of water in a cloud must in- 

 crease its total specific gravity. 



16. While, therefore, the heat given out in the precipitation 

 of watery particles tends to expand the air and diminish its den- 

 sity, a counter effect is to some extent produced by the weight 

 of those particles. It is not possible to determine whether pre- 

 cipitation will really increase or decrease the density of the 

 cloudy air, and accelerate or arrest its upward motion, except 

 by exactly calculating out the amounts of the two opposite effects, 

 and thus discovering in which direction the resultant tends. A 



