248 Mr Elackadder on circumstances connected with the 
It is rare to have this motion of the air rendered thus visible 
on low plains, and so near the surface ; but something very 
much the same may more frequently be seen from mountains 
high enough to command a downward view of the clouds which 
form in the evening at the upper boundary of a lower stratum 
of air, that is incumbent over extensive low plains, in which ve- 
getation is luxuriant. 
In a former part of these remarks, similar temporary and lo- 
cal agitations of the air, when otherwise in a calm state, were 
found perfectly to account, as it is believed, for the increase of 
temperature indicated by a thermometer lying on snow ; and, on 
the present occasion, it enables us equally satisfactorily to ex- 
plain the condensation of vapour on polished metals, after sun- 
set, and at a time when hygroscopic and similar substances have 
suffered a considerable depression of temperature below that of 
the air a small distance above them. 
Two circumstances may here be adverted to, though, after 
what has been already said, their explanation presents no diffi- 
culty : ItStf, On the occasions referred to, polished metals never 
have their temperatures much reduced; and the quantity of 
moisture condensed on their surface from that cause is never very 
considerable. 2d, The surface of a polished metal is sometimes 
observed to become obscured by the condensation of vapour, 
and shortly afterwards again brilliant from the re-evaporation of 
t,he^ moisture, at a time when hygroscopic and other similar sub- 
stances seem to suffer no change in regard to moisture. The 
least quantity of moisture condensed on a polished surface, 
appearance (if it did not proceed from the unusual irritation which the light re- 
flected from such clouds always communicates to the eye), and that twinkling of 
the stars so much more apparent at one time than at another, are produced by si- 
milar agitations of the air, at no great distance from the earth ? Two strata of air, 
in different conditions, in regard to heat and moisture, often come into contact. On 
some such occasions a cloud is produced, and on others rain or snow ; depending, 
it is presumed, on the relative conditions of the two bodies of air, and the degree 
of mechanical force with which they are brought into contact, or blended. But 
there are doubtless occasions when the conditions of two or more contiguous strata 
are such that the warmer communicates heat to the colder, without any deposition 
of moisture ; and, on these occasions, such an agitation may take place at the 
plains of intermixture, as to produce the appearance of a vibratory motion, or 
twinkling of bodies situated at a distance. 
