IRR 
IRR 
tlie place pointed out where a cut, C, should 
be made, whereby the whole of the inclined 
plane, U, might be irrigated to the greatest 
advantage ; the surplus-water draining off 
into the lower level of still- water, E, from 
which it would not he possible to raise the 
water to the superior parts of the inclined 
plane, CD, without the aid of expensive 
machinery. This section will, we trust, 
prove completely satisfactory, by sSewiiig 
how necessary it is to look back to superior 
levels, often within reach. 
Under the head of compound irrigation, 
we consider the various changes of direc- 
tion, attended with an iiitermixture of tiie 
several modes laid down for simple irriga- 
tion. In the former, we occasionally find 
th.e w’ater caught several times by tlie same 
stream, which, being obstructed at its seve- 
ral turns by weirs, sluices, &c., enables us 
to abbreviate the succession of ridges. This 
is a matter of great importance, because it 
renders a less body of water, in the branch- 
drains of the first level, equal to every pur- 
pose, and obviates the mischief that some- 
times attends upon a numerous succession 
of levels, when the quantity of water re- 
quired for the whole is forced through the 
first, in which, by its weight and volume, 
the roots of the grass are denudated, and 
the finer parts of the soil completely wash- 
ed away. It is far better to give the stream 
a second, or even a third, turn ^through the 
land, than to allow all the water, necessary 
to moisten six or seven successive levels, to 
pass through the first. A reference to fig. 9, 
will give some idea of this mode ; by the 
courses of the dotted lines, and arrows, the 
various descents may be understood. 
With respect to the season for watering 
laud, so many varieties prevail, in conse- 
quence of soil, and of locality, tliat we can 
only observe, in general terms, that, where 
lauds are to be inundated completely, by 
letting the water assume an unlimited 
range, and to expand over all parts which 
come under its level, such places require, 
during the winter season, to be kept well 
covered, that the frost may not attack the 
plants while saturated with moisture: if 
that were to happen, the whole would be 
destroyed ; whereas, by a periodical inunda- 
tion the grass is sheltered from frost; and, 
by drawing off the water as the spring ad- 
vances, and at intervals of about ten days, 
when the weather is fair, such grass will 
shoot out vigorously, and afford a very early 
bite for cattle, at that season when green 
food is both valuable and scarce. The sanje 
principle may be followed, though the prac- 
tice is different, in places watered by 
drains. In such, the greatest care ought to 
be taken to avoid throwing on the water 
while the air is frosty ; but so soon as the 
weather opens, the ground ought to be mo- 
derately moistened. The sun’s power should 
guide us to the frequency and quantity of 
water ; nor should its quality be overlook- 
ed : water from warm soils wifi produce ef- 
fects widely ditt'ereiit from the streams flow- 
ing out of clay lands, or such as are impr eg- 
nated with iron, &c. The best water 
usually rises out of gravelly or chalky lands. 
It is better to throw the water on early in 
the day, during cold weather, in order that 
the grass may dry well, and the danger ap- 
prehended from frosty nights be obviated ; 
but in suinmei', the watering should take 
place late in the evening, whereby the 
ground will be cold, without danger of 
scorching the plants. 
We have dwelt thus tong on the subject 
of irrigation, under the conviction of its ex- 
treme importance : the reader may, under 
the head of Agriculture, find a few addi- 
tional remarks, which were given witli the, 
view to bringing all matters relating to 
farming under one general head, while we 
reserved this mechanical part to be sepa- 
rately treated, under its proper designa- 
tion. 
IRRITABILITY, in physiology, is tlie 
property peculiar to the muscles, by which 
they contract upon the application of certain 
stimuli, without a consciousness of action. 
Haller and other physiologists denominate 
that part of the human body irritable, which 
becomes shorter by being touched; very 
irritable, if it contracts upon a slight touch. 
They call that a sensible part of the human 
body, which, upon being touched, trans- 
mits the impression of it to the mind : on 
the contrary, they call that insensible, which 
being burnt, torn, cut, &c. occasions no 
sign of pain or convulsion, nor any sort of 
change in tlie situation of the body. It. is 
inferred that the epidermis' is insensible ; 
that the ti'ue skin is the most sensible part 
of the body ; that the fat and cellular mem- 
brane are insensible ; and the muscular flesh 
sensible, the sensibility of which he ascribes 
rather to the nerves than the flesh itself. 
The tendons, having no nerves distributed 
among them, are deemed insensible. Irri- 
tability then is the distinguishing charac- 
teristic between the muscular and cellular 
fibres. Irritability differs from sensibility, 
and is not proportioned to it: the intestines 
