THU 
fliinating 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, we shall be at no loss to ac- 
count for the extraordinary operations vul- 
garly ascribed to thunderbolts. As stones 
and bricks struck by lightning are often 
found in a vitrified state, we may reason- 
ably suppose, with Beccaria, that some 
stones in the earth, having been struck in 
this manner, gave occasion to tlie vulgar 
opinion of the thunderbolt. 
Thunder-clouds are those clouds which 
are in a state fit for producing lightning and 
thunder. From Beccaria’s exact and cir- 
cumstantial account of the external appear- 
ances of thr.nder-clouds, the following par- 
ticulars are extracted. The first appear- 
ance of a thunder storm, which usually hap- 
pens when there is little or no wind,'is one 
dense cloud, or njore, increasing very fast 
in size, and rising into the higher regions of 
the air. The lower surface is black and 
nearly level ; but the upper finely ai died, 
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 arches. At the time of the rising of 
this cloud, the atmosphere is commonly full 
of a great iifany separate clouds, that are 
motionless, hut} of odd whimsical shapes. 
All these, upon the appearance of the thun- 
der-cloud, draw towards it, and become 
more uniform in their shapes as they ap- 
proach ; till, coming very near the thunder- 
cloud, their limbs mutually stretch towards 
one another, and they immediately coalesce 
into one uniform mass. These he calls ad- 
scititious clouds, from their coming in to 
enlarge the size of the thunder-cloud. But 
sometimes the thunder-cloud will swell, and 
increase very fast, without the conjiiction 
of any adscititions clonds ; the vapours in 
the atmosphere forming themselves into 
clouds wherever it passes. Some of the ad- 
scititioHS clouds appear like white fringes, 
at tlie skirts of the thunder-cloud, or under 
the body of it, but they keep continually 
growing darker and darker, as they ap- 
proach to unite with it. When the thun- 
der-cloud is grown to a great size, its lower 
surface is often ragged, particular parts be- 
ing detached towards the earth, but still 
connected with the rest. SoraetiniM the 
THY 
lower surface swells into various large pro- 
tuberances bending uniformly downward ; 
and sometimes one whole side of the cloud 
will have an inclination to the earth, and 
the extremity of it nearly touch the ground. 
. When the eye is under the thunder-cloud, 
after it is grown larger, and well formed, it 
is seen to sink lower, and to darken prodi- 
giously ; at the same time that a number 
of small adscititious clouds (the origin of 
which can never be perceived) are seen in 
a rapid motion, driving about in very un- 
certain directions under it. While these 
clouds are agitated with the most rapid 
motions, the rain commonly falls in the 
greatest plenty, and if the agitation be ex- 
ceedingly great, it commonly hails. While 
the thunder-cloud is swelling, and extend- 
ing its branches over a large tract of coun- 
try, the lightning is seen to dart from one 
part of it to another, and often to illumi- 
nate its whole mass. When the cloud has 
acquired a sufficient extent, tlie lightning 
strikes between the cloud and the earth, in 
two opposite places, the path of the light- 
ning lying through the whole body of the 
cloud and its branches. The longer this 
lightning continues, the less dense does the 
cloud become, and the less dark its appear- 
ance ; till at length it breaks in different 
places, and shows a clear sky. These thun- 
der-clouds were sometimes in a positive as 
well as a negative state of electricity. The 
electricity continued longer of the same 
kind, in proportion as the thunder-cloud 
was simple and uniform in its direction ; 
but when the lightning changed its place, 
there commonly happened a change in the 
electricity of the apparatus over which the 
clonds passed. It would change suddenly 
after a very violent flash of lightnmg, but 
the change w'ould be gradual when the 
lightning was moderate, and the progress of 
the thunder-cloud slow. See Priestley’s 
History of Electricity. 
THYMBRA, in botany, a genus of the 
Didynamia Gymnospermia class and order. 
Natural order of Verticillatae or Lahiatie. 
Essential character: calyx sub-cylindricaly 
two-lipped, scored on each side with a vil- 
lose line ; style semibifid. There are three 
species. 
THYMUS, in botany, thtjme, a genus of 
the Didynamia' Gyumosirermia class and or- 
der. Natural order of Verticillatae or La- 
biatae. Essential character: throat of the 
two-lipped calyx closed with villose hairs. 
There are twenty- two species. 
THYNNUS, in natural history, a genus 
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