THUNDER. 
the continent, also in onr country in Corn- 
wall. 
THUNBERGIA, in botany, so named 
in honour of Charles Peter Thunberg, M. D. 
professor of botany in the university of 
Upsal, &c. &c. a genus of the Didynaraia 
Angiospermia class and order. Natural or- 
der of Personatae. Acanthi, Jussieu. Es- 
sential character ; calyx double ; outer two- 
leaved j inner twelve-toothed ; corolla bell- 
shaped ; capsule beaked, two-celled. There 
are two species, viz. T. capeusis, and T. 
fragians. 
THUNDER, 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. 
The rattling in the noise of thunder, which 
makes it seem as if it passed through arches, 
is probably owing to the sound being ex- 
cited among clouds hanging over one ano- 
ther, and the agitated air passing irregularly 
between them. 
The explosion, if high in the air, and re- 
mote from us, will done mischief; but when 
near, it may, and has in a thousand in- 
stances, destroyed trees, animals, &c. This 
proximity, or small distance, may be esti- 
mated nearly by the interval of time be- 
tween seeing the flash of lightning, and 
hearing the report of the thunder, estimat- 
ing the distance after the rate of 1,142 feet 
per second of time, or 3§ seconds 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 very near us, 
and even among ns. And in such cases, 
the doctor assures us, he has sometimes 
foretold the mischiefs that happened. 
Although in this country thunder may 
happen at any time of the year, yet the 
months of July and Aug\ist are those in 
which it may almost certainly be expected. 
Its duration is of very uncertain continu- 
ance ; sometimes only a few peals will be 
heard at any particular place during the 
whole season ; at other times the storm will 
return at the interval of three 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 place 
so frequently in any year, but in many sea- 
sons it will be perceptible tliat thunder 
clouds are formed in the neighbourhood, 
even at these short intervals. Hence it ap- 
pears, that during this particular period 
there must be 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 wea- 
ther, for we have often a long tract of hot 
weather without any thunder ; and besides, 
though not common, thunder is sometimes 
heard in the winter also. As therefore the 
heat of the weather is common to the whole 
summer, whether there be thunder or not, 
we mtist look for the causes of it in tliose 
phenomena, whatever they are, which are ^ 
peculiar to the months of July, August, 
and die beginning of September. Now it 
is generally observed, that from the month 
of April, an east or south-east wind gene- 
nerally takes place, and continues with lit- 
tle interruption till towards the end of June. 
At that time, sometimes sooner and some- 
times later, a westerly wind takes place ; but 
as the causes producing the east wind ar« 
not removed, the latter opposes the west 
wind with its whole force. At the place of 
meeting, there is naturally a most vehement 
pressure of the atmosphere, and friction of 
its parts against one another ; a calm en- 
sues, and the vapours brought by both 
winds begin to collect and form dark 
clouds, wliicli.can have little motion either 
way, because they are pressed almost equal- 
ly on all sides. For the most part, how- 
ever, the west wind prevails, and what lit- 
tle motion the clouds have is towards the 
east : whence the common remark in this 
country, that “ thunder-clouds move 'against 
the wind.” But this is by no means univer- 
sally true ; for if the west wind hajtpens to 
be excited by any temporary cause before < 
its natural period when it should take place, 
the east wind will very frequently get the 
better of it ; and the clouds, even although 
thnnder is produced, will move westward. 
Yet in either case the motion is so slow, 
that the most superficial observers cannot 
help taking notice of a considerable resist- 
ance in the atmosphere. 
When lightning acts with extraordinary 
violence, and breaks or shatters any thing, 
it is called a thunderbolt, which the vulgar, 
to fit it for such eifects, suppose to be a 
hard body, and even a stone. But that we 
need not to have recourse to a hard solid 
body to account for the effects commonly 
attributed to the thunderbolt, will be evi- 
dent to any one, who considers those of 
gunpowder, and the several chemical ful- 
