Thus It IS evident that, in the latter month, the atmosphere contains four inches of water
more than in the former month. Hence, supposing the usual iutcrmixtiu'c of currents
of air, 111 both the intervening periods, to be tho same, the rain ought to be four inches
less in the former period of tlie year than tlie average, and four inches more in tlie latter
period, making a difference of eight inclies between the two periods, which nearly accords
■with the preceding obseiwations.
1391. Frost, being derived from the atmosphere, naturally proceeds from the upper parts
oi bodies downwards ; so, the longer a frost is continued, tlie thicker the ice becomes
upon tlie ivater in ponds, and the deeper into the earth the ground is frozen. In about
sixteen or seventeen days’ frost, Boyle found it had penetrated fourteen inches into the
ground. A t Moscow, in a hai-d season, the frost will penetrate two feet deep into the
ground ; and Captain James found it penetrated ten feet deep in Charlton Island, and
the water in the same island was frozen to the depth of sLx feet. Scheffer assures us,
that, in fewedcn, the frost pierces three feet (a Swedish ell) into the earth, turning what
moistm-e is found there into a whitish substance, like ice ; and into standing water three
ells or more. Tlie same author also mentions sudden cracks or rifts in the ice of the
lakes of Sweden, nine or ten feet deep, and many leagues long ; the iiipture being made
with a noise not less loud than if many guns were discharged together. By such means
however, the fishes are furnished with air, so that they are rarely found dead. ’
1392. The history o f frosts furnishes vciy extraordinary facts. The frees are often
scorched and bm-nt up, as with the most excessive heat, in consequence of the separation
of water from the air, which is therefore veiy diying. In the great frost in 1683 the
trunks of oak, ash, walnut, and other trees, were miserably split and cleft, so that thev
might be seen tlirough, and the cracks often attended with dreadful noises like the ex
plosion of fire-ai-ms. In the frost of 1837—8 lai-ge bushes of heath had their stems split
by the frost into shi-cds, and the wood of the evergreen oak and that of the sweet W
was cracked and split in a similar manner.
1393. H ail is generally defined as frozen rain ; it differs from it in that the hailstones
tor the most part m-e not fonncd of single pieces of ice, but of many little spherules
agglutinated togetlier ; neither ai-e those sphci-ules all of the same consistence ; some of
them being hai-d and solid, like perfect ice ; others soft, and mostly like snow hardened
by a severe frost. Hailstones have sometimes a kind of core of this soft matter ; but more
frequently the core is solid and hard, whüe the outside is formed of a softer matter.
Hailstones assume various figm-cs, being sometimes round, at other times pyramidal
crenated, angulai-, thin or flat, and sometimes steUated with sLx radu, like the small
co stá is of snow. Natural historians ftirnish us -with various accounts of surprising
showers of haü,_ m which the hailstones were of extraordinary magnitude.
1394. Nnow is formed by the freezing of the vapom-s in the atmosphere. I t differs
from haü and hoar frost, in being, as it were, crj-stallised, whüe they are not. As the flakes
tall down through the atmosphere, they ai-e continually joined by more of these radiated
spiciüa, and they increase in bulk like the drops of rain or haüstones. The lightness of
snow, although it is firm ice, is owing to the excess of its sm-face in comparison with the
matter contained under it : as gold itself may be extended in sm-face tü l it will ride upon
the le ^ t breath of air. The whiteness of snow is owing to the small particles into which
It is divided ; for ice, when pounded, wül become equally white.
1395. Snow is o f great use to the vegetable kingdom. Were we to judge from appearance
only, we imglit imagine, that, so far fi-om being useful to the earth, the cold humidity of
snow would be detrimcntal to vegetation : but the experience of all ages asserts the con-
fraiy. bnow, particularly m those northern regions where the ground is covered with
It tor several months, fructifies the eaith, by guai-ding the corn or other vegetables from
the intenser cold of the au-, and especially from the cold piercing winds. I t has been
a vulgar opimon, very generally received, that snow fertüises the land ou which it faUs
more than ram, in consequence of the nitrous salts which it is supposed to acquire by
freezing : but it appears from the experiments of Margraaf, in the year 1731, that the
cnemical difference between ram and snow-water is exceedingly small ; that the latter
contains a somewhat less proportion of earth than the former; but neither of them
contains either earth, or any kind of salt, in any quantity which can be sensibly efficacious
in promoting vegetation. The peculiar agency of snow as a fertiliser, in prc-
ierence to rmn, may be ascribed to its furnishing a covering to the roots of vegetables
by which they are guarded fi-om the influence of the atmospherical cold, and the
internal heat of the earth is prevented from escaping. Different vegetables are able to
preserve hfe under different degrees of cold, but all of them perish when the cold which
reaches then- roots is extreme. Proridcnce has, therefore, in the coldest climates pro-
vided a covenng of snow îor the roots of vegetables, by which they are protected from
the Mu en o e of the atmospherical cold. The snow keeps in tho internal heat of tho
em-th, which suiTounds the roots of vegetables, and defends them from the cold of the
atmosphere.
1396. Ice is water in the solid state, during wliich the temperature remains constant,
being 32° of the scale of Fahrenheit. Ice is considerably lighter than water, namely,
about one eighth part ; and this increase of dimensions is acquired with prodigious force,
sufficient to bm-st the strongest iron vessels, and even pieces of artüleiy. Congelation
takes place much more suddenly than the opposite process of liquefaction ; and, of course,
the same quantity of heat must be more rapidly extricated in freezing than it is absorbed
in thawing ; the heat thus extricated being disposed to fly off in all directions, and little
of it being retained by the neighbouring bodies, more heat is lost than is gained by the
alternation : so that where ice has once been formed, its production is in this manner
redoubled.
1397. The northern ice extends during summer about 9° from the pole ; the southern
18° or 20°; in some parts even 30°; and floating ice has occasionally been found in
both hemispheres as far as 40° from the poles, and sometimes, as it has been said, even in
latitude 41° or 42°. Between 54° and 60° south latitude, the snow lies on the ground,
at the sca-side, throughout the summer. The line of p c^ e tu a l congelation is three raües
above the surface at the equator, where the mean heat is 84°; at Tenerife, in latitude
28°, two miles ; in the latitude of London, a little more than a mile ; and in latitude 80°
north, only 1250 feet. A t the pole, according to the analogy deduced by Kirwaii, from
Mayer’s Formula, and which is not however found to agree very exactly with what takes
place, from a comparison of irarious observations, the mean temperature should be 31°.
In London the mean temperature is 50°; at Rome and at Montpelier, a little more than
60°; in the island of Madeira, 70°; and in Jamaica, 80°.
1398. Wind. Were it not for this agitation of the afr, putrid effluvia arising from the
lialiitatioiis of man, and from vegetable substances, besides the exhalations fi-om water,
wonld soon render it unfit for respiration, and a general mortality would be the consequence.
The prevailing winds of our own countiy, which were ascertained by order of
the Royal Society of London, a t London, ai-e,
W in d s . D a y s .
South-west - 112
North-east - 58
North-west - 50
W in d s. D a y s .
West - - 53
South-east - 32
East - - 26
W in d s . D a y s ,
South - - 18
North - - 16
The westerly winds blow more upon an average in each month of the year than any
other, pai-ticularly in Ju ly and A u g u st; the north-cast wind prevails during January,
March, April, May, and June, and is most unfrequent in Febmary, July, September!
and December; the north-west occun-ing more fi-equently from November to March,
and less so in September and October than in any other months.
1399. Near Glasgow, the average is stated as follows : —
W in d s.
South-west
North-west
D a y s .
174
40
North-east
South-east
104
47
1400. In Ireland, the prevailing winds are the west and south-west.
1401. The different degrees o f motion o f wind next excite oui* attention; and it seems
almost superfluous to obsci-ve, that it varies in gradation fr-om the mildest zejiliyr, which
plays upon the leaves of plants, gently undulating them, to the furious tempest, calculated
to inspire horror in tlie breast of the most callous. I t is also a remai-kable fact,
that violent cm-rcnts of air pass along, as it were, within a line, without sensibly agitating
that beyond them. An instance of the fury of the wind being bounded “ by a line ”
occurs ill the huiTicane of America; where its devastating com-se is often accurately
marked in the forests for a great extent in one direction.
1402. Causes o f wind. There arc many circumstances attending the operations of
tlio afr, which we tei-m wind, which serve as a basis for well-founded conjectures, and
those, united to the result of daüy observation, render the explanation of its phenomena
tolerably satisfactoiy.
1403. I t must be clear to the most common capacity, that as the rays of the sun descend
perpendicularly on the surface of the earth under tho torrid zone, that part of it must
receive a greater proportion of heat than those parts where they fall obliquely; the heat
thus acquired communicates to the afr, which it rarefies, and causes to ascend, and the
vacuum occasioned by this operation is immediately filled by the chill afr from the north
and south. The dim-nal motion of the earth gradually lessens to the poles from the
equator, at which point it moves at the rate of fifteen geogi-apliical mües in a minute,
aud this motion is communicated to the atmosphere in the same degree; but if part of
the atmosphere were conveyed instantaneously to the equator fi-om latitude 30°, it would
not dfrectly acquire the equatorial velocity; consequently, the ridges of the earth must
meet it, and give it the appearance of an cast wind. The effect is similar upon the cold
air proceeding from the north and south, and the similarity must be admitted to extend