35 6 SCIENCE PROGRESS 



view. How far the anomalies are due to the fact that the 

 determination of the gradient may be at fault, on account of 

 minute local differences of pressure, is a question to which at 

 present a full answer cannot be given. Land and sea breezes 

 at the surface near the coast, which have apparently no relation 

 to the barometric gradient, may perhaps be connected with local 

 differences of pressure not shown on an ordinary chart (i). 



The gradient wind is doubtless different at different levels 

 because the quotient gradient divided by density may change as 

 we go up. Nor can we assume that the actual wind is neces- 

 sarily represented by the gradient wind, even when the local 

 variations of pressure are allowed for. There are meteorological 

 conditions in which the scheme of pressure distribution is chang- 

 ing and the steady state is disturbed by causes of which we 

 cannot specify the details. Hence, above our heads, we may have 

 a criss-cross of currents some of which our local observations 

 will account for ; the causes of others are beyond our cognisance. 



The study of the motion of clouds often gives evidence of 

 currents at different levels in different directions and the ob- 

 servations with pilot balloons show us that there are crossing 

 currents at different levels without any clouds to indicate them. 

 The pilot balloon would be invisible if there were clouds ; the ob- 

 servations of pilot balloons and clouds are thus complementary. 



Let us go on to consider some local peculiarities in the 

 structure of the atmosphere. We have seen that there are 

 layers of different velocity at different levels. We cannot sup- 

 pose that the layers are always kept distinct. The air of 

 one layer may pass upward through a layer above, in con- 

 sequence of some unstable distribution of density due perhaps 

 to warming or cooling either by conduction or dynamically. 

 The cumulus cloud is itself evidence of rising air; the visible 

 condensation of the vapour is in fact attributed to that cause. 



The study of the floating of clouds would be a very instruc- 

 tive exercise for a balloonist, because the causes which produce 

 or vary the ascent of what we may picture as a large globe of air 

 are included in those which affect the balloon. The balloon has 

 others in addition, so that the case of a floating cloud or mass of 

 air is simpler than that of a balloon. But the case of a cloud 

 requires some close consideration. 



To give an illustration, imagine a layer of detached clouds 

 floating in the middle of a layer of clear air 2 kilometres 



