in a Voyage across the Atlantic. 133 
ones. Condensed vapour, sinking through an atmosphere ca- 
pable of dissolving it, will yet fall partially below the precise 
level at which the hygrometer denotes it to be soluble: some 
portion, be it little or much, escapes solution, and comes down 
as rain. So vapour, rising into an atmospheric stratum capable 
of condensing it, is not all instantaneously condensed; a por- 
tion of it, little or much, passes. For both rising and falling 
vapour obeys the impulse of levity and weight more promptly 
than it relinquishes or acquires heat. 
Relative height of drifting clouds, which has been spoken 
of, is to be inferred from the velocity with which they drift, as 
already mentioned: or that of any clouds is to be judged of 
according to the distinctness with which the commotion of their 
vapour is perceivable. For every cloud consists of transient 
materials; and, though seemingly a definite mass, viewed dis- 
tantly and cursorily, it is, in truth, continually varied, momenta- 
rily receiving accessions on one part, and wasting on another ; 
as is manifest when it is steadily contemplated. 
The absolute height of a cloud above the sea, is in like man- 
ner to be estimated from the clearness with which the intestine 
commotion of its vapour is distinguishable, assisted by a habit 
of observation acquired amidst mountains of known altitude. 
Indeed, most of the phenomena of clouds require to be studied 
among mountains; being best explained by reference to appear- 
ances presented where they are most accessible. I shall, there- 
fore, not enlarge on the subject of them as seen from a position 
whence they are least attainable. 
K 2 
