442 
P L A 
P L A 
P L A 
to which the Moon is fixed, slides up and 
down through a hole in the end of the arm ; 
and the lower end of the wire rests on a cir- 
cular ring v, whose plane is parallel to the 
plane of the Moon’s orbit; so that as the 
arm turns round, the wire is pushed up by 
the inclination of the ring, and falls by its own 
weight. Beneath this is a ring with divisions 
on it, shewing the Moon’s age. When the 
winch L is turned, it works the worm-wheel 
beneath tire board M, and moves the frame 
N (fig. 1) with the Earth round the Sun; 
and as the wheel g is fixed, the wheel h is 
turned by rolling round it ; and as k (which 
it works) is of tiie same size with the other 
two, it turns the Earth so. that its axis always 
points to the pole. The wheel l, by means 
of the wheel o, turns the pinion which car- 
ries Mercury B (tig. 1). The wheel m with the 
wheel p turns g, and the planet Venus D (fig. 
1) ; and the wheel u turns the pinion s', and 
the Moon as before described. The number 
of teeth in the wheels and pinions respec- 
tively, must be so proportioned, that each 
planet may revolve in the same periods as 
we have described under Astronomy. 
PLANIMETRY, that part of geometry 
which considers lines and plane figures, with- 
out any regard to heights or depths. Plani- 
metry is particularly restricted to the mensu- 
ration of planes and other surfaces ; as con- 
tradistinguished from stereometry, or the 
mensuration of solids, or capacities of length, 
breadth, and depth. Planimetry is formed 
by means of the squares of long measures, 
as square inches, square feet, square yards, 
&c. that is, by squares whose side is au inch, 
a foot, a yard, &c. So that the area or con- 
tent of any surface is said to be found, when 
it is known how many such square inches, 
feet, yards, &c. it contains. 
PLANISPHERE, signifies a projection 
of the sphere, and its various circles on a 
} >lane ; in which sense maps, wherein are ex- 
libited the meridians, and other circles of 
the sphere, are planispheres. See Map, 
Projection, &c. 
Planisphere, is more particularly used 
for an astronomical instrument used in ob- 
serving the motions of the heavenly bodies. 
It consists of a projection of the celestial 
sphere upon a plane, representing the stars, 
constellations, &c. in their proper , order; 
some being projected on the meridian, and 
others on the equator. 
PLANT AGO, plantain; a genus of 
the monogyuia order, in the tetrandria class 
of plants. The calyx is four-cleft; corolla 
four-cleft; border reflex ; stamina very long ; 
capsules two-celled, cut transversely. There 
are 38 species, of which the most noted are: 
1. The common broad-leaved plantain, called 
weybread ; 2, the great hoary plantain, or lamb’s 
tongue; 3. the narrow-leaved plantain, or 
ribwort; and the following varieties have 
also been found in England, which are acci- 
dental; the besom plantain and rose-plantain. 
'Pile plantains grow naturally in pastures in 
most parts of England, and are frequently 
very troublesome weeds. The common 
plantain and ribwort-plantain are both used 
In medicine, and are so well known as to 
need no description. They are said to be 
slightly astringent ; and the green leaves are 
commonly applied to fresh wounds by the 
■common people. 
Of the coronopus,br buck’s-horn plantain, 
there are two varieties growing in England, 
viz. the common buck’s-horn, which grows 
plentifully on heaths every where ; and the 
narrow-leaved Welsh sort, which is found 
upon many of the Welsh mountains. The 
first of these was formerly cultivated as a 
salad herb in gardens, but has been long ba- 
nished for its rank disagreeable flavour; it is 
sometimes used in medicine. 
PLANTING, in agriculture and garden- 
ing. The first thing in planting is to prepare 
the ground before the trees or plants are taken 
out of the earth, that they may remain out 
of the ground as short a time as possible, 
and the next is to take up the trees or plants 
in order to their being transplanted. In tak- 
ing up the trees, carefully dig away the earth 
round the roots, so as to come at their seve- 
ral parts to cut them off ; for if they are torn 
out of the ground without care, the roots 
will be broken and bruised to the great in- 
jury of the trees. When you have taken 
them up, the next thing is to prepare them 
for planting by pruning the roots and heads. 
And first, prune off all the bruised or broken 
roots, all such as are irregular, and cross 
each other, and all downright roots, especi- 
ally in fruit-trees ; shorten the larger roots 
in proportion to the age, the strength, and 
nature of the tree : observing that the wal- 
nut, mulberry, and some other tender-rooted 
kinds, should not be pruned so close as the 
more hardy sorts of fruit and forest-trees ; in 
young fruit-trees, such as pears, apples, 
plumbs, peaches, &c. that are one year old 
from the time of their budding or grafting, 
the roots may be left only about eight or 
nine inches long ; but in older trees they 
must be left of a much greater length : but 
this is only to be understood of the larger 
roots ; for the small ones must be chiefly cut 
quite out, or pruned very short. The next 
thing is the pruning of their heads, which 
must be differently performed in different 
trees; and the design of the trees must also 
be considered : thus, if they are designed for 
walls or espaliers, it is best to plant them with 
the greatest part of their heads, which should 
remain on till they begin to shoot in the 
spring, when they must be cut down to five 
or six eyes, at the same time taking care not 
to disturb the roots. But if the trees are de- 
signed for standards, you should prune off all 
the small branches dose to the place where 
they are produced, as also the irregular ones 
which cross each other; and after having 
displaced these branches, you should also cut 
off all such parts of branches, as have by any 
accident been broken or wounded ; but by 
no means cut off the main leading shoots 
which are necessary to attract from the root, 
and thus promote the growth of the tree. 
Having thus prepared the trees for planting, 
you must now proceed to place them in the 
earth ; but if the first trees have been long out 
of the ground, so that the fibres of the roots are 
dried, place them eight or ten hours in water 
before they are planted, with their heads 
erect, and the roots only immersed in it ; 
which will swell the dried vessels of the roots, 
and prepare them to imbibe nourishment 
from the earth. In planting them, great 
regard should be had to the nature of the 
soil; for if that is cold and moist, the trees 
should be planted very shallow ; and if it is a 
hard rock or gravel, it will be better to raise 
a hill of earth where each tree is to be plant* 
| ed, than to dig into the rock or gravel, and 
! fill it up with earth, as is too often practised, 
; by whic h means the trees are planted as in a 
tub, and have but little room to extend their 
roots. The next thing to be observed is, 
to place the trees in the hole in such a man- 
ner, that the roots may be about the same 
depth in the ground, as before they were 
taken up ; then break the earth fine with a 
spade, and scatter it into the hole, so that it 
may fall in between every root, that there 
may be no hollowness in the earth. Having 
filled up the whole, gently tread down the 
earth with your feet, but do not make it too 
j hard ; which is a great fault, especially if the 
i ground is strong or wet. Having thus plant- 
ed the trees, they should be fastened to stakes 
driven into the ground, to prevent their being 
displaced by the wind, and some mulch laid 
about the surface of the ground about their 
roots : as to such as are planted against walls, 
their roots should be placed about a foot 
from the wall, to which their heads should 
be nailed to prevent their being blown up 
1 by the wind. The seasons for planting are 
various, according to the different sorts of 
trees, or the soil in which they are planted ; 
for the trees whose leaves fall off in winter, 
the best time is the beginning of October, 
provided the soil is dry ; but if it is a very 
wet soil, it is better to defer it till the latter 
end of February, or the beginning of March ; 
and for many kinds of evergreens, the begin- 
ning of April is by far the best season ; 
though they may be safely removed at Mid- 
summer, provided they are not to be carried 
very far; but you should always make choice 
of a cloudy wet season. 
PLANTS, physiology of. The constitu- 
ent or, elementary principles of vegetables 
in general, are hydrogen, oxygen, and char- 
coal. These, as far as our observations have 
hitherto extended, are common to all vege- 
tables. There are some other substances, 
such as lime, potass, iron, and azote, which 
are occasionally found in vegetables ; but as 
they are not common to all plants, they can- 
not be considered as essential to the consti- 
tution of vegetable matter. 
The parts of vegetables which naturalists 
are accustomed to consider as distinct in their 
nature and functions, are six : the stem or 
trunk, the root, the leaf, the flower, the fruit, 
and the seed. In many vegetables the root 
appears nearly similar, in all its constituent 
parts and principles, to the stem or trunk, 
and indeed the one seems a continuation of 
the other ; which must be our apology for 
reversing in some degree the order of nature, 
and treating first of that part ; which, though 
it seems to proceed or spring immediately 
from the other, is yet the most perfect in its 
'"organization, and is in general of the greatest 
use and importance to man. 
I. Tne stem or trunk, which includes also 
the branches, we might say all the more solid 
and substantial parts of a tree or plant, con- 
sists of three parts, the bark, the wood, and 
the pith. 
1st. The bark is protected on the outside 
by a cuticle, epidermis, or scarf-skin, which 
consists sometimes of numerous layers, and 
differs in thickness in different plants. This 
skin or cuticle is an organized body, corti- 
posed of very minute bladders, interspersed 
with longitudinal fibres, as in the nettle, 
thistle, and the generality of herbs. It con- 
