90 Mr Blackadder on Circumstances connected with the 
thermometers suspended in the air, and lying on the grass, may 
proceed from the passage of a moister body of air, of a higher 
temperature, part of the aqueous vapour being condensed into 
a cloud at its upper boundary. 
Fully to illustrate this view of the subject, it would be requi- 
site to enter upon a wide field, still requiring cultivation. For 
there are abundant reasons for believing, that the formation of 
clouds is a subject still very imperfectly understood. 
At present I shall only remark, 1st, That, on the occasions re- 
ferred to, the cloud is always connected with the lower stratum of 
air, and the increase of temperature is always most apparent when 
the cloud is comparatively low and dense. %d, That, when the 
cloud is high, and unconnected with the lower stratum of air, 
no change of temperature is observed to take place. 3d, That 
the change from a lower to a higher, and from that again to a 
lower, temperature, always infers a progressive motion of the air. 
The body of air over the place of observation is not stationary, 
its place being occupied by other bodies of air which pass in 
succession. 4 th, That the increased temperature, if not in- 
fluenced by the passage of a more heated body of air, never 
exceeds, and but seldom equals, that of the ground. 5th, That, 
during the increase in the temperature of the air, there is a de- 
crease of the cold caused by evaporation ; and the change in the 
latter usually greatly exceeds that in the former. 6ih, That, when 
the temperature of bodies at the surface of the earth has been 
observed to increase during the passage of a cloud, the moisture 
of the air has also been observed to increase, by means of a hy- 
groscopic hygrometer *. From these and other considerations, 
* An expansion hygrometer of extreme sensibility, may be constructed, by ar- 
ranging a number of sentient slips in a form similar to the strings of a harp, but of 
equal lengths, and so connected, that the united expansions and contractions of the 
whole shall be pointed out by an index. One instrument of this kind, which I 
had constructed, and which was left with a friend on the Continent, possessed 
great sensibility ; its range comprehending only two ordinary hygrometric degrees, 
though its scale was several inches in length ; but by a simple contrivance, its in- 
dex could be readily adjusted to any degree of a common hygrometer. Such an 
instrument is obviously unfit for the more usual purposes of hygrometers, but it is 
admirably fitted for indicating slight or transient changes in the state of atmos- 
pheric humidity. An instrument of this kind has been observed to continue for 
