MET 
M I C 
commences ■, and the lower clouds, arriving 
from the windward, move under this sheet, 
and are successively lost in it. When the 
latter cease to arrive, or when the sheet 
breaks, every one’s experience teaches him 
to expect an abatement or cessation of rain. 
But there otten follows, what seems hitherto 
to have been unnoticed, an immediate and 
great addition to the quantity of cloud. For 
on the cessation of rain, tire lower broken 
clouds which remain rise into cumuli, and the 
superior sheet puts on tire various forms of 
the cirro-stratus, sometimes passing to the 
cirro-cumulus. 
!f the interval is long before the next 
shower, the cumulo stratus usually makes its 
appearance, which it also does sometimes 
very suddenly after the first cessation. 
But we see the nature of this process more 
perfectly, in viewing a distant shower in pro- 
file. 
It tire cumulus be the only cloud present at 
such a time, we may observe its superior part 
to become tufted with cirri. Several adjacent 
clouds also approach, and unite laterally by 
subsidence. 
The cirri increase, extending themselves 
upward and laterally; after which the shower 
is seen to commence. At other times tire 
converse takes place of what has been de- 
scribed relative to the cessation of rain. The 
cirro-stratus is previously formed above the 
cumulus; and their sudden union is attended 
with the production of cirri and rain. 
In either case the cirri vegetate, as it were, 
in proportion to the quantity of rain falling ; 
and give the cloud a character by which it 
is easily known at great distances, and to 
which, in the language of meteorology, we 
may appropriate tire nimbus of the Latins : 
Qualis ubi ad terras abrupto sidere nim- 
bus 
It mare per medium ; miseris, heu ! prescia 
longe 
Horrescunt corda agricolis. — Virgil. 
When one of these arrives hastily with the 
wind, it brings but little rain; and frequently 
some hail or driven straw. In heavy showers 
the central sheet, once formed, increases to 
windward, the cirri being propagated above 
and against the lower current, while the cu- 
muli, arriving with the latter, are successively 
arrested in their course, and contribute to 
reinforce the shower. 
In continued gentle rains it does not ap- 
pear necessary, for the resolution of the 
clouds, that the different modifications should 
come into actual contact. It is sufficient, 
that there exi-t two strata of clouds, one 
passing beneath the other, and each conti- 
nually tending to horizontal uniform diffu- 
sion. It will ram during this state of the two 
strata, although they should be separated by 
an interval of many hundred feet in eleva- 
tion. 
As the masses of cloud are always blended, 
and their arrangement destroyed, before rain 
eomes on, so tire reappearance of those is the 
signal for its cessation. The thin sheets of 
cloud which pass over during a wet day, cer- 
tainly receive from the humid atmosphere a 
supply proportionate to their consumption; 
wlrile the latter prevents their increase in 
bulk Hence a seeming paradox, which yet 
■accords strictly with observation ; that for 
any given hour of a wet day, or any given 
MIC 
day of a wet season, the more cloud the less 
t ain. Hence also arise some further reflec- 
tions on the purpose answered by clouds in 
tire economy of nature. Since rain may be 
produced by, and continue to fall from, the 
slightest obscuration of the sky, by the nim- 
bus, that is, by two sheets in different states, 
while the cumulus, or cumuio-stratus, with 
the most dark and threatening aspect, passes 
over without letting fall a drop, until their 
change of state commences; it should seem 
that the latter are the reservoirs, in which the 
water is collected from a large space of at- 
mosphere, for occasional and local irrigation 
in dry seasons, and try means of which it is also 
arrested at times in its descent, in the midst 
of wet ones. In this so evident provision 
for l lie sustenance of all animal and vegetable 
life, as well as for the success of mankind in 
that pursuit so essential to their welfare, in 
temperate climates, of cultivating the earth, 
we may discover the wisdom and goodness of 
the Creator and Preserver of all things. 
The nimbus, although in itself one of the 
least beautiful clouds, is yet now and then 
superbly decorated with its attendant, the 
rainbow, which can only be seen in perfection 
when backed by the widely extended uniform 
gloom of this modification. 
METHOD, in logic, <kc. the arrange- 
ment of our ideas in -such a regular order, 
that their mutual connection and dependance 
may be readily comprehended. 
METONYMY, in rhetoric, is a trope in 
which one name is put for another, on account 
of the near relation there is between them. 
By this trope any of the most significant cir- 
cumstances of a thing are put for the thing 
itself. See "Rhetoric. 
METOPE. See Architecture. 
METRE, in poetry. See Hexameter, 
Pentameter, &c 
METROSIDEROS, a genus of the class 
and order icosandria monogynia. The calyx 
is five-cleft, half-superior ; petals five ; sta- 
mina very long, standing out; stigma sim- 
ple ; capsule three-celled. There are 13 
species, of New Holland, &c. 
MEZEREON. See Daphne. 
MEZZOTINTO. See Engraving. 
MIASMA, among physicians, denotes the 
contagious effluvia of pestilential diseases, 
whereby they are communicated to people 
at a distance. 
MICA. This stone forms an essential part 
of many mountains, and has been long known 
under the names of glacies Marine, and Mus- 
covy glass. It consists, of a great number 
of thin laminae adhering to each other, some- 
times of a very large size. Specimens have 
been found in Siberia nearly 2\ yards square. 
It is sometimes crystallized; its primitive 
form is a rectangular prism, whose bases are 
rhombs with angles of 120° and 60° : its inte- 
grant molecule has the same form. Some- 
times it occurs in rectangular prisms, whose 
bases also are rectangles, and sometimes also 
in short six-sided prisms ; but it is much more 
frequent in plates or scales of no determi- 
nate figure or size. 
Its texture is foliated. Its fragments flat. 
The lamellae flexible, and somewhat elastic. 
Very tough. Often absorbs water. Specific 
gravity from 2.6546 to 2.9342. Feels smooth, 
but not greasy. Powder feels greasy. Co- 
lour, when purest, silver white or grey ; but 
it occurs also yellow, greenish, reddish" 
brown, and black. Mica is fusible by the 
blowpipe into a white, grey, green, or black 
enamel ; and this last is attracted by the mag- 
net. Spanish wax rubbed by it becomes 
negatively electric, 
A specimen of mica, analysed by Vauque- 
lin, contained 
50.00 silica 
35.00 alumina 
7.00 oxide of iron 
1.35 magnesia 
1.33 lime. 
94.68 v 
Mica has long been employed as a substi- 
tute for glass. A great quantity of it is said 
to be used in the Russian marine for panes to 
the cabin-windows of ships; it is preferred, 
because it is not so liable as glass to be broken 
by the agitation of the ship. It is also used 
in our navy for lantherns, tor the use of the 
powder-rooms. 
MICIIELIA, a genus of the octandria 
polygynia class of plants, the flower of w hich 
consists of eight petals; the fruit consists of a 
number of globose unilocular berries, dis- 
posed in a cluster ; in each of which there 
are four seeds, convex on one side, and angu- 
lar on t he other. ’There are two species, 
trees of the East Indies. 
MICHAUXIA, a genus of the class and 
order octandria monogynia. The calyx is 
16- parted; corolla wheel-shaped, 8 -parted; 
nect. 8-vaIved, staminiferous ; caps. 8-celled, 
many-seeded. There is one species, a bi- 
ennial of Aleppo, resembling the campanula. 
MICROMETER, an astronomical ma- 
chine, which, by means of a screw, serves to 
measure extremely small distances in the hea- 
vens, &c. and that to a great degree of ac- 
curacy. 
The micrometer consists of a graduated 
circle (Plate Miscel. fig. 162), of a screw qo , 
and its index qr. The threads of the screw 
are such, that 50 make the length of one 
inch exactly. When it is to be used, the 
point o is set to the side of the part to be 
measured, and then the index is turned about 
with the linger, till the eye perceives the 
point has just passed over the diameter of 
that part; then the number of turns, and 
parts of a turn, shewn by the graduated cir- 
cle, will give the dimensions in parts of an 
inch, as we shall shew by the following ex- 
ample: Suppose it is required to measure the 
diameter of a human hair, and I observe the 
index is turned just once round while the 
point o passes over it ; then it is plain the 
diameter of the hair ip the image is -J_th of 
an inch. Now if the microscope, IDE F,. 
d ef, magnifies 6 times, or makes the image 
6 times larger in diameter than the object, 
then is the diameter of the hair itself but 
-Jth of -Jg., that is, but ^igtli, part of an inch. 
Also it is to be observed, that as there are 
ten large divisions, and twenty small ones,, 
on the micrometer plate, so each of those 
small divisions is the ^L-th of -^th, or the 
To Vg tl 1 part of an inch. Therefore, if, in 
measuring any part of an object, you observe 
how many of these smaller divisions are pass- 
ed over by the index, you will have so many 
thousandth parts of an inch for the .qj^asuie 
required.. 
