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to each place particularly heated by the beams of the sun. The moon, being a Large
body situated comparatively near the earth, is known to affect the atmosphere ; and this,
and the continual shifting of the point of the earth’s surface over which the sun is vcr-
ticiil, to ^le west, ai-c given as the causes of the tides and of the trade-winds. The moon’s
revolutioiis, by pressing the atmosphere upon the sea, cause the flux and reflux which
wc call tides ; it cannot, therefore, be doubted, that some of the winds we experience
are caused by the moon’s motion.
1404. The regular motion o f the atmosphere, known by the name of landand sea breezes,
may be explained by the effects of rarefaction : the air heated over the land rises uj),
because rarefied, and its place is supplied by the cooler air which flows in from tlie sea ;
tliis produces the sea breeze; at sunset, the equilibrium is first restored ; but as the eai'th
cools faster by radiation than the water, the air over it becomes cooler than that over the
sea, especially if there be mountains in the vicinity ; the air over the laud then displaces
the light afr from the sea, and thus the land breeze is formed. Granting that the attraction
of the moon, and the diurnal movement of the sun, affect our atmosphere, there
cannot be a doubt but a westwai'd motion of the air must prevail within the bounclai-ics
of the trade-winds, the consequence of which is an easterly cuiTcnt on each side : from
this, then, it proceeds that south-west winds ai-c so frequent in the western paits of
Europe, and over the Atlantic Ocean. IGi'wan atti'ibutes our constant south-west winds,
particularly during winter, to an opposite cuiTcnt prevailing between the coast of Malabar
and the Moluccas at the same period : this, he adds, must be supplied from regions
close to the pole, which must be recmitcd in its tui*n from the countries to the south of
it, ill the western parts of our hemisphere.
1405. The variable winds cannot be so readily accounted for ; yet it is evident that,
though they seem the effects of capricious causes, they depend upon a régulai* system,
an-anged by the great Author of nature. That accm-ate and successful observer of pai-t
of his works, the celebrated Franklin, discovered, in 1740, that winds originate at the
precise points towai-ds which they blow. This philosopher had hoped to observe an eclipse
of the moon at Philadelphia, but was prevented by a north-east stoi-m, that commenced
at seven in the evening. This he afterwards found did not occur at Boston till eleven ;
and, upon inquiry, he had reason to suppose it passed to the north-east at the rate of
about 100 miles an hour. The manner in which ho accounts for this reti-ogradc proceeding
is so satisfactory, that wc shall give it in his own words, pai-ticulai-ly as his
assertions are supported by recent observations, both in America and Scotland. He
argued thus :—“ I suppose a long canal of water, stopped at the end by a gate. The
water is at rest till the gate is opened ; then it begins to move out tln-ougli the gate, and
the water next the gate is put iu motion and moves on towai-ds the gate ; and so on
successively, till the water at the head of the canal is in motion, which it is last of all. In
this case all the water moves indeed towards the gate ; hut the successive times of beginning
the motion are in the contrary way, viz. from the gate back to the head of the
canal. Thus, to produce a north-east storm, I suppose some great rarefaction of the air
in 01- near the Gulf of Mexico ; the ah- rising thence has its place supplied by the next
more northern, cooler, and therefore denser and heavier air ; a successive current is
formed, to which our coast and inland mountains give a north-east direction.” According
to the obseiwations made by Captain Cook, the north-east winds prevail in the
Northern Pacific Ocean during the same spring months they do with us, from which
facts it appeai-s the cold afr from America and the north of Europe flows at that season
into the Pacific and Atlantic Oceans.
1406. Other descriptions o f winds may ai-ise from a vai-iety of causes. The atmosphere
has been ascertained to be composed of afr, vapour, and carbonic acid and water;
and as it is well known that these frequently change their aerial foi-m, and combine with
different substances, and the reverse, consequently partial winds and accumulations must
continuaUy occur, which occasion winds of different degrees of violence, continuance,
and direction.
1407. The principal electrical phenomena o f the atmosphere are thunder and lightning.
1408. Thunder is the noise occasioned by the explosion of a flash of lightning passing
through the air; or it is that noise which is excited by a sudden explosion of electrical
clouds, which are therefore called thunder-clouds.
1409. The rattling, in the noise of thunder, whieh makes it seem as if it passed tlirough
arches, is probably owing to the sound being excited among clouds hanging over one
another, between wliich the agitated air passes irregulai-ly.
1410. The explosion, if high in the air and remote from us, will do no mischief, but
when near, it may; and it has, in a thousand instances, destroyed trees, animals, &c.
This proximity, or smaU distance, may be estimated nearly by the inteiwal of time
between seeing the flash of lightning, and hearing the report o fth e thunder, reckoning
the distance after the rate of 1142 feet to a second of time, or four seconds and a half
to the mile. Dr. Wallis observes, that commonly the difference between the two is
about seven seconds, which, at the rate above mentioned, gives the distance almost two
miles: but sometimes it comes in a second or two, which argues the explosion vci-y near
to us, and oven among us ; and in such cases, the doctor assures us, he has sometimes
foretold the mischiefs that happened.
1411. Season o f Thunder. Although in this countiy thunder may happen at any
time of the yeai*, yet tlie months of July and August are those in which it may ahnost
certainly be expected. Its visitations are of very uncertain continuance ; sometimes
only a few peals will be heard at any particular place during the whole season ; at other
times the stoi*m will return, at intervals of tliree or four days, for a month, six weeks, or
even longer ; not that we have violent thunder in this country directly vertical in any
one plaBe so frequently in any year, but in many seasons it will be perceptible that
thunder-clouds are foi-med in the neighbourhood, even at these short inteivals. Hence
it appears, that, during this particular period, there must he some natural cause
operating for the production of this phenomenon, which does not take place at other times.
This cannot be the mere heat of the weather, for wc have often a long succession of
hot weather without any thunder ; and, besides, though not common, thunder is sometimes
heard in the winter. As therefore the heat of tho weather is common to the whole
summer, whether there is thunder or not, wo must look for the causes of it in those
phenomena, whatever they are, which are peculiar to the months of July, August, and
the beginning of September. Now, it is generally obscn-ed, that from the month of April,
an east or south-east wind generally takes place, and continues with little intcn-uption
tiU towards the end of June. At that time, sometimes sooner, and sometimes later, a
westerly wind takes place ; but as the causes producing the east wind are not removed,
the latter opposes the west wind with its whole force. At the place of meeting, there arc
naturally a most vehement pressure of the atmosphere, and friction of its parts against
one another ; a calm ensues, and the vapours brought by both winds begin to collect and
form dark clouds, which can have little motion cither way, because they are pressed almost
equ^y on all sides. For the most pait, however, tlie west wind prevails, and what little
motion the clouds have is towards the east : whence the common remark in this countiy,
that “ thunder-clouds move against the wind.” But this is by no means universally tme :
for if the west wind happens to be excited by any temporary cause before the natural
period when it should take place, the east wind will very fi-cquently get the better of it ;
and the clouds, even although thunder is produced, will move westward. Yet in either
case, the motion is so slow, that the most supei-ficial obseiwers cannot help taking notice
of a considerable resistance in the atmosphere.
1412. Thunderbolts. When lightning acts with cxtraordinaiy violence, and breaks or
shatters any thing, it is called a thunderbolt, which the vulgai*, to fit it for such effects,
suppose to be a hai-d body, and even a stone. But that we need not have recourse to a
hard solid body to account for the effects commonly attributed to the thunderbolt, will
be evident to any one who considers those of gunpowder, and the several chemical fulminating
powders, but more especially the astonishing powers of electricity, when only
collected and employed by human art, and much more when directed and exercised in
the course of nature. When we consider the known effects of electrical explosions, and
those produced by lightning, wc shall be at no loss to account fur the extraordinai-y
operations vulgarly ascribed to thunderbolts. As stones and bricks stmck by lightning
are often found in a vitrified state, wc may reasonably suppose, with Beccaria, that some
stones in the earth, having been strack iu this manner, gave occasion to the common
opinion of the thunderbolt. Some of tlie small smooth flhits which ai-e shown as
thunderbolts appear to have been ancient arrow-heads.
1413. Thunder-clouds are those clouds which are in a state fit for producing lightning
and thunder. The first appearance of a thunder-stoi-m, which usually happens wlien
there is little or no wind, is one dense cloud, or more, increasing very fast in size, and
rising into the higher regions of the air. Tlie lower surface is black, and ncai-ly level ;
but the upper finely arched, and well-defined. Many of these clouds often seem piled
upon one another, all arched in the same manner; but they are continually uniting,
swelling, and extending their ai-ches. At the time of the rising of this cloud, the
atmosphere is commonly full of a great many scpai-ate clouds, which are motionless, and
of odd whimsical shapes ; aU these, upon the appcai-ance of the thunder-cloud, draw
towards it, and become more uniform in thefr shapes as they approach ; till, coming vciy
near the thunder-cloud, their limbs mutually stretch towards one another, and they
immediately coalesce into one unifoi-m mass. Sometimes the thunder-cloud will swell,
and increase very fast, without the conjunction of any adjoining clouds ; the vapours in
thé atmosphere forming themselves into clouds whenever it passes. Some of the additional
clouds appear like white fringes, at the skirts of the thunder-cloud, or under the
body of it ; but they keep continually growing darker and darker, as they approach to
unite with it. When the thuiidcr-cloud is grown to a great size, its lower surface is often
ragged, particular parts being detached towai'ds the eai-tli, but still connected with tho
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