hich falls at different parts of the earth's
surface is very variable; and for the purpose of measuring it instruments
called _Rain-gauges_ have been contrived. The simplest form is a funnel
three or four inches high, and having an area of one hundred square
inches. This may be placed in the mouth of a large bottle, and, after
each fall of rain, the quantity may be measured by a glass jar divided
into inches and parts. This simple gauge being placed on the ground in
an open spot, will evidently represent a portion of the ground, and will
show the depth of rain which would cover it at and about that spot,
supposing the ground to be horizontal, and that the water could neither
flow off nor sink into the soil. Thus, by taking notice of the quantity
of rain which falls day by day, and year by year, and taking the average
of many years, we get the mean annual quantity of rain for the particular
spot in question. By an extension of these observations, it is evident
that the mean annual fall of rain may be known for a district or a
kingdom.

A more convenient form of rain-gauge than the one just noticed, is made
by placing the funnel at the top of a brass or copper cylinder, connected
with which at the lower point, is a glass tube with a scale, measuring
inches and tenths of an inch. The water stands at the same height in the
glass tube as it does in the cylinder, and being visible in the tube the
height can be immediately read on the scale. The cylinder and the tube
are so constructed, that the sum of the areas of their sections is a
given part, such as a tenth of the area of the mouth of the funnel; so
that each inch of water in the tube is equal to the tenth of an inch of
water which enters the mouth of the funnel. A stop-cock is added for
drawing off the water from the cylinder after each observation is noted
down.

Some rain-gauges are constructed for showing the quantity of rain which
falls from each of the four principal quarters. Others are made so as to
register, themselves, the quantity of rain fallen. One of this kind, by
Mr. Crosley, consists of a funnel through which the rain passes to a
vibrating trough; when, after a sufficient quantity has fallen into its
higher side, it sinks down and discharges the rain which escapes by a
tube. The vibrating action of this trough moves a train of wheel-work
and indices, which register upon a dial plate the quantity of rain
fallen.

Whatever form of rain-gauge is