adopted, it must be placed in an exposed
situation, at a distance from all buildings, and trees, and other objects
likely to interfere with the free descent of rain into the funnel. It is
usual, in rainy weather, to observe the quantity of water in the gauge
every morning; but this does not seem to be often enough, considering how
freely water evaporates in an exposed situation. An error may also arise
from some of the water adhering to the sides of the vessel, unless an
allowance is made for the quantity thus lost by a contrivance such as the
following:--Let a sponge be made damp, yet so that no water can be
squeezed from it, and with this collect all the water which adheres to
the funnel and cylinder, after as much as possible has been drawn off;
then, if the sponge be squeezed, and the water from it be received in a
vessel which admits of measuring its quantity, an estimate may be made of
the depth due to it; and this being added to the depth given by the
instrument, would probably show correctly the required depth of rain.

When snow has fallen the rain-gauge may not give a correct quantity, as a
portion of it may be blown out, or a greater quantity may have fallen
than the mouth will contain. In such cases, it is recommended to take a
cylindrical tube and press it perpendicularly into the snow, and it will
bring out with it a cylinder equal to the depth. This, when melted, will
give the quantity of water which can be measured as before. The
proportion of snow to water is about seventeen to one; and hail to water,
about eight to one. These quantities, however, may vary according to the
circumstances under which the snow or hail has fallen, and the time they
have been upon the ground.

The rain-gauge should be placed as near the surface of the ground as
possible; for it is a perplexing circumstance, that the rain-gauge
indicates very different quantities of rain as falling upon the very same
spot, according to the different heights at which it is placed. Thus it
has been found, that the annual depth of rain at the top of Westminster
Abbey was 12.1 inches nearly, while, on the top of a house sixteen feet
lower, it was rather more than 18.1 inches, and on the ground, in the
garden of the house, it was 22.6 inches. M. Arago has also found from
observations made during twelve years, that on the terrace of the
Observatory at Paris the annual depth was about 2.25 inches less than in
the court thirty yards below.