A dictionary of modern gardening - University Library

H YD 319 H YG
column, forcing up the pulse-valve,
Practice shows that a ten-feet
shuts it.
fall vi'ill raise a column of water one
hundred and fitly feet high, at the rate
of five quarts per minute, or one part
raised to eleven wasted, where the ram
is only supplied by a two-inch pipe. I
may further add, that theory teaches
that a ten-feet fall will raise water three
hundred feet high— of course, in a very
small quantity. Mr. II. P. M'Birkinbrine,
of Philadelphia, has been very
successful in the construction of this
valuable power.
IIYUROCHAPJS morsmran(r. Hardy
aquatic. Seed and runners. Stillwater.
HYDROLEA. Two species. One
stove evergreen shrub, and the other
stove herbaceous. Cuttings. Loam
and peat.
HYDROPELTIS purpurea. Halfhardy
aquatic. Offsets. Still water.
HYGROMETER is an instrument
deserving of employment in the stove,
green house, and conservatory nearly
as much as the thermometer ; for the
correct degree of dampness of the air is
of very great importance in the cultivation
of plants, and scarcely less than
that of the temperature in which they
vegetate.
The perspiration from the leaves of
plants increases with the air's dryness,
and decreases w-ith its moistness. If it
be excessive, not only are their juices
too much reduced, but the very texture
of the leaves is destroyed. If, on the
other hand, tlie perspiration is prevented,
the juices are too watery, and the
secretions and assimilations arc devoid
of consistency, rendering the plants too
succulent and weak.
" It is impossible for any one to know
what degree of moisture he really maintains
in a forcing-house without an in-
strument by which to measure it :
that
instrument is the hygrometer, which
might as well be called the 'watergauge,'
which is what the first word
really means. Of the many contrivances
to effect this end, the best for all
practical purposes, is Daniell's Hygrometer,
of which the annexed cut (Fig.
93) exhibits the general appearance.
It measures the moisture in the air
quickly and precisely, and is not subject
to get out of order.
" If moisture is brought into contact
with a substance sufficiently cold, a part
of the moisture is condensed, and is so
converted from a state of invisible vapour
into water.
" Thus, in a cold day, the glass roof
of a green-house may be seen streaming
with water, which runs down and forma
'drip;' and in this often unsuspected
manner air is rendered dry, notwithstanding
the operations of syringing,
steaming, &c. Daniell's Hygrometer is
constructed with reference to this circumstance.
The figure represents two
hollow glass balls containing ether, and
communicating by the glass tube which
rests on the support. The ball which
forms the termination of the longer
leg is of black glass, in order that the
formation of dew on its surface may be
the more perceptible. It includes the
bulb of a delicate thermometer dipping
in the ether, its scale being inclosed in
the tube above the ball ; and whatever
change takes place in the temperature
of the ether is indicated by this thermometer.
The other ball is covered with
muslin. In making an observation it is
first necessary to note down the temperature
of the air ; next turn the instrument,
so that when the muslin-covered
ball is held in the hand, the ether may
escape into the blackened ball ; and it
should also be held till the included
thermometer rises a few degrees above
the temperature of the air, when it
should be replaced on the support.
Then drop, or gently pour, a little ether
on the muslin. The evaporation of this
extremely volatile substance produces
cold ; and attention must be instantly
directed to the black glass ball and included
thermometer. The latter will
be seen falling rapidly ; and at length
a ring of dew will appear at the line
which runs across the black ball —
quickly, if the air is very moist, slowly,
if the air is dry. If the air is very dry,
no moisture will be thus deposited till
the thermometer falls to, perhaps, 10",
20^, or 303 below the temperature of
the air. But at vv'hatever temperature
the dew forms, that temperature should
be noted as the dew-point; and the difference
between it and the temperature
of the air, at the time, is the degree of
dryness according to the indications of
this hygrometer. Thus, in a moderately
dry day, let it be supposed that the
temperature of the air is 6')^ in the
shade, and that the muslin requires to
be kept moist, before dew is formed,
till the blackened ball containing the