BALLOON ASCENTS. 
413 
of the Pyrenees ; and by the time they have crossed that 
mountainous country^ they are deprived of so much of their 
high temperatures^ that Prance can derive comparatively but 
little heatj if any_, from them_, and hence^ apparently^ is due 
her cold winters. One other fact I may refer to in respect to 
the circumstance met with on that day^ it is the presence of 
cold snow above the warm air. Such a state of things is quite 
sufficient to account for the production of any amount of fog, 
extending over any amount of surface ; and it may be that the 
simultaneous appearance of dense fogs at times over the whole 
country, and even extending far out to sea, may be attributable. 
Till the present time, I have never been able to account at all 
for the production of such extensive fogs. 
A useful inference may be drawn from the fact of hot and 
cold currents of air of different velocities and extent existing 
at the same time, in contact with each other. Such a state 
is but a struggle between forces, either of which may prepon- 
derate at any moment ; and to this cause may be attributed, 
with high probability, the all but unaccountable changes that 
so suddenly take place at times during winter, and clearly 
indicates that how warm soever the weather may be at this 
season of the year, the warm current of air may be deflected 
at any moment, and give place to the then dominant cold 
current, for which we should at all times be prepared. 
It deserves consideration, whether the secular increase in 
the temperature of the air in England is not due to these 
anti-trade winds ; whether, in fact, the currents of air which 
constitute the equatorial streams of England have not, during 
the present century, continually been increasing in strength 
and duration, and the continuation of the increase set in 
a hundred years ago probably may be greatly dependent on 
the corresponding change of force of the trade-winds them- 
selves. 
Respecting the results of the researches connected with the 
varying amount of water in the air, at different elevations, it 
may be the most convenient mode to express in the terms of 
humidity of the air, considering that the whole amount possible 
at the then temperature be represented by 100, and determining 
how many such parts were really present — the more or less 
dryness of the air, at the different elevations, will thus be 
made apparent : 100 showing complete wetness, there being' 
present then all possible, the number 50 will show that one- 
half only of the quantity of water required for saturation was 
really present, and so on, extreme dryness being shown by 
very small numbers. 
Thus treated, when the sky was cloudy, it was found that 
on the ground the degree of humidity was 74 j increased to 
