METHOD OF MEASURING THE QUANTITY OF RAIN THAT FALLS--THE RAIN
GAUGE--METHODS OF OBSERVING FOR RAIN AND SNOW--EFFECTS OF ELEVATION ON
THE QUANTITY OF RAIN--DIFFERENCE BETWEEN THE TOP OF A TALL BUILDING AND
THE SUMMIT OF A MOUNTAIN--SIZE OF DROPS OF RAIN--VELOCITY OF THEIR
FALL--QUANTITY OF RAIN IN DIFFERENT LATITUDES--EXTRAORDINARY FALLS OF
RAIN--REMARKS ON THE RAIN OF THIS COUNTRY--INFLUENCE OF THE MOON--ABSENCE
OF RAIN--REMARKABLE DROUGHT IN SOUTH AMERICA--ITS TERRIBLE EFFECTS AND
CONSEQUENCES--ARTIFICIAL RAINS.
The quantity of rain which 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 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.
It would naturally be expected from these observations, that less rain
falls on high ground than at the level of the sea. Such however is not
the case, except on abrupt elevations; where the elevation is made by the
natural and gradual slope of the earth's surface, the quantity of rain is
greater on the mountain than in the plain. Thus, on the coast of
Lancashire, there is an annual fall of 39 inches; while at Easthwaite,
among the mountains in the same county, the annual depth of rain amounts
to 86 inches. By comparing the registers at Geneva and the convent of
the Great St. Bernard, it appears that at the former place, by a mean of
thirty-two years, the annual fall of rain is about 30.75 inches; while at
the latter, by a mean of twelve years, it is a little over 60 inches.
In order to explain these remarkable differences, it must not be supposed
that the clouds extend down to the ground, so as to cause more rain at
the foot of Westminster Abbey than on its roof. There is no doubt that
in moist weather the air contains more water near the ground than a few
hundred feet above it; and probably, the same cause which determined a
fall from the cloud, would also throw down the moisture floating at a low
elevation. Much rain also proceeds from drifting showers, of short
duration, and the current moves more slowly along the surface, and allows
the drops to fall as fast as they are formed. In hilly countries, on the
contrary, clouds and vapours rest on the summits without descending into
the plains, and, according to some, the hills attract electricity from
the clouds, and thus occasion rain to fall. Mr. Phillips supposes that
each drop of rain continues to increase in size from the commencement to
the end of its descent, and as it passes successively through the moist
strata of the air, obtains its increase from them; while the rain which
falls on the mountain may leave these moist strata untouched, so that
they may, in fact, not form rain at all.
The drops of rain are of unequal size, as may be seen from the marks made
by the first drops of a shower upon any smooth surface. They vary in
size from perhaps the twenty-fifth to a quarter of an inch in diameter.
It is supposed that in parting from the clouds they fall with increasing
speed, until the increasing resistance of the air becomes equal to their
weight, when they continue to fall with an uniform velocity. A
thunder-shower pours down much faster than a drizzling rain. A flake of
snow, being perhaps nine times more expanded than water, descends thrice
as slow. But hailstones are often several inches in length, and fall
with a velocity of seventy feet in a second, or at the rate of about
fifty miles an hour, and hence the destructive power of these missiles in
stripping and tearing off fruit and foliage.
The annual quantity of rain decreases from the equator to the poles, as
appears from the following table, which gives the name of the station,
its latitude, and the average annual number of inches of rain:--
Coast of Malabar lat. 11 degrees 30' 135.5 inches.
N.
At Grenada, Antilles 12 degrees 126
At Cape Francois, St. 19 degrees 46' 120
Domingo
At Calcutta 22 degrees 23' 81
At Rome 41 degrees 54' 39
In England 50 to 55 degrees 31
At St. Petersburgh 59 degrees 16' 16
At Uleaborg 65 degrees 30' 13.5
The number of rainy days, on the contrary, increases from the equator to
the poles.
From 12 to 43 degrees N. lat.--the number of rainy days in 78
the year amounts to
From 43 to 46 degrees 103
From 46 to 50 degrees 134
From 50 to 60 degrees 161
The greatest depth of rain which falls in the Indian ocean is during the
time when the periodical winds, called the _monsoons_, change their
direction. When the winds blow directly in-shore the rains are very
abundant, so much so that, after a continuance of twenty-four hours, the
surface of the sea has been covered with a stratum of fresh water, good
enough for drinking, and ships have actually filled their casks from it.
Colonel Sykes observes, that the deluge-like character of a monsoon in
the Ghats of Western India, is attested by the annual amount of 302.25
inches, at Malcolmpait, on the Mahabuleshwar Hills.
A great depth of rain in a short time has occasionally been witnessed in
Europe. At Genoa, on the 25th of October, 1822, a depth of thirty inches
of rain fell in one day. At Joyeuse, on the 9th of October, 1827,
thirty-one inches of rain fell in twenty-two hours. Previous to the
great floods of Moray, in 1829, the rain is described as being so thick
that the very air itself seemed to be descending in one mass of water
upon the earth. Nothing could withstand it. The best finished windows
were ineffectual against it, and every room exposed to the north-east was
deluged. The smaller animals, the birds, and especially game, of all
kinds, were destroyed in great numbers by the rain alone, and the mother
partridge, with her brood and her mate, were found chilled to death
amidst the drenching wet. It was also noticed, that, as soon as the
flood touched the foundation of a dry stone wall, the sods on the top of
it became as it were alive with mice, all forcing their way out to escape
from the inundation which threatened their citadel; and in the stables,
where the water was three feet deep, rats and moles were swimming about
among the buildings.
Among the Andes it is said to rain perpetually; but in Peru it never
rains, moisture being supplied during a part of the year by thick fogs,
called _garuas_. In Egypt, and some parts of Arabia, it seldom rains at
all, but the dews are heavy, and supply with moisture the few plants of
the sandy regions.
There is a great variation in the quantity of rain that falls in the same
latitude, on the different sides of the same continent, and particularly
of the same island. The mean fall of rain at Edinburgh, on the eastern
coast, is 26 inches; while at Glasgow, on the western coast, in nearly
the same latitude, it is 40 inches. At North Shields, on the eastern
coast, it is 25 inches; while at Coniston, in Lancashire, in nearly the
same latitude, on the western coast, it is 85 inches.
The amount of rain in a district may be changed by destroying or forming
forests, and by the inclosure and drainage of land. By thinning off the
wood in the neighbourhood of Marseilles, there has been a striking
decrease of rain in fifty years.
In Mr. Howard's observations on the climate of this country, he has
found, on an average of years, that it rains every other day; that more
rain falls in the night than in the day; that the greatest quantity of
rain falls in autumn, and the least in winter; that the quantity which
falls in autumn is nearly double that in spring; that most rain falls in
October and least in February, and that May comes nearest to the mean:
that one year in every five, in this country, may be expected to be
extremely dry, and one in ten extremely wet.
According to Dalton, the mean annual amount of rain and dew for England
and Wales is 36 inches. The mean all over the globe is stated to be 34
inches.
There seems to be some real connexion between the changes of the moon and
the weather. Mr. Daniell says, "No observation is more general; and on
no occasion, perhaps, is the almanac so frequently consulted as in
forming conjectures upon the state of the weather. The common remark,
however, goes no further than that changes from wet to dry, and from dry
to wet, generally happen at the changes of the moon. When to this result
of universal experience we add the philosophical reasons for the
existence of tides in the aerial ocean, we cannot doubt that such a
connexion exists. The subject, however, is involved in much obscurity."
At Viviers, it was observed that the number of rainy days was greatest at
the first quarter, and least at the last. Mr. Howard has observed that,
in this country, when the moon has south declination, there falls but a
moderate quantity of rain, and that the quantity increases till she has
attained the greatest northern declination. He thinks there is "evidence
of a great _tidal wave_, or swell in the atmosphere, caused by the moon's
attraction, preceding her in her approach to us, and following slowly as
she departs from these latitudes."
Most dry climates are subject to periodical droughts. In Australia, they
return after every ten or twelve years, and are then followed by
excessive rains, which gradually become less and less till another
drought is the consequence.
When Mr. Darwin was in South America, he passed through a district which
had long been suffering from dry weather. The first rain that had fallen
during that year was on the 17th of May, when it rained lightly for about
five hours. "With this shower," he says, "the farmers, who plant corn
near the sea-coast, where the atmosphere is more humid, would break up
the ground; with a second, put the seed in; and, if a third should fall,
they would reap in the spring a good harvest. It was interesting to
watch the effect of this trifling amount of moisture. Twelve hours
afterwards the ground appeared as dry as ever; yet, after an interval of
ten days, all the hills were faintly tinged with green patches; the grass
being sparingly scattered in hair-like fibres a full inch in length.
Before this shower every part of the surface was bare as on a high road."
A fortnight after this shower had fallen, Mr. Darwin took an excursion to
a part of the country to which the shower had not extended. "We had,
therefore," he says, "in the first part of our journey a most faint tinge
of green, which soon faded away. Even where brightest, it was scarcely
sufficient to remind one of the fresh turf and budding flowers during the
spring of other countries. While travelling through these deserts, one
feels like a prisoner, shut up in a gloomy courtyard, longing to see
something green, and to smell a moist atmosphere."
The effects of a great drought in the Pampas are thus described. "The
period included between the years 1827 and 1830 is called the 'gran seco'
or the great drought. During this time so little rain fell, that the
vegetation, even to the thistles, failed; the brooks were dried up, and
the whole country assumed the appearance of a dusty high road. This was
especially the case in the northern part of the province of Buenos Ayres,
and the southern part of St. Fe. Very great numbers of birds, wild
animals, cattle, and horses, perished from the want of food and water. A
man told me that the deer used to come into his courtyard to the well
which he had been obliged to dig to supply his own family with water; and
that the partridges had hardly strength to fly away when pursued. The
lowest estimation of the loss of cattle in the province of Buenos Ayres
alone, was taken at one million head. A proprietor at San Pedro had
previously to these years 20,000 cattle; at the end not one remained.
San Pedro is situated in the midst of the finest country, and even now
again abounds with animals; yet, during the latter part of the 'gran
seco' live cattle were brought in vessels for the consumption of the
inhabitants. The animals roamed from their _estancias_, and wandering
far to the southward, were mingled together in such multitudes that a
government commission was sent from Buenos Ayres to settle the disputes
of the owners. Sir Woodbine Parish informed me of another and very
curious source of dispute; the ground being so long dry, such quantities
of dust were blown about, that in this open country the landmarks became
obliterated, and people could not tell the limits of their estates.
"I was informed by an eye-witness, that the cattle in herds of thousands
rushed into the river Parana, and being exhausted by hunger they were
unable to crawl up the muddy banks, and thus were drowned. The arm which
runs by San Pedro was so full of putrid carcasses, that the master of a
vessel told me, that the smell rendered it quite impossible to pass that
way. Without doubt, several hundred thousand animals thus perished in
the river. Their bodies, when putrid, floated down the stream, and many
in all probability were deposited in the estuary of the Plata. All the
small rivers became highly saline, and this caused the death of vast
numbers in particular spots, for when an animal drinks of such water it
does not recover. I noticed, but probably it was the effect of a gradual
increase, rather than of any one period, that the smaller streams in the
Pampas were paved with bones. Subsequently to this unusual drought, a
very rainy season commenced, which caused great floods. Hence it is
almost certain, that some thousands of these skeletons were buried by the
deposits of the very next year. What would be the opinion of a geologist
viewing such an enormous collection of bones, of all kinds of animals and
of all ages, thus embedded in one thick earthy mass? Would he not
attribute it to a flood having crept over the surface of the land, rather
than to the common order of things?"
Captain Owen mentions a curious effect of a drought on the elephants at
Benguela on the western coast of Africa:--"A number of these animals had
some time since entered the town in a body to possess themselves of the
wells, not being able to procure any water in the country. The
inhabitants mustered, when a desperate conflict ensued, which terminated
in the ultimate discomfiture of the invaders, but not until they had
killed one man, and wounded several others." The town is said to have a
population of nearly three thousand. Dr. Malcolmson states, that during
a great drought in India the wild animals entered the tents of some
troops at Ellore, and that a hare drank out of a vessel held by the
adjutant of the regiment.
In connexion with droughts may be mentioned a plan {133} proposed by Mr.
Espy of the United States of America, for remedying them by means of
_artificial rains_. That gentleman says, that if a large body of heated
air be made to ascend in a column, a large cloud will be generated, and
that such cloud will contain in itself a self-sustaining power, which may
move from the place over which it was formed, and cause the air over
which it passes to rise up into it and thus form more cloud and rain,
until the rain may become general.
It is proposed to form this ascending column of air by kindling large
fires which, Mr. Espy says, are known to produce rain. Humboldt speaks
of a mysterious connexion between volcanoes and rain, and says that when
a volcano bursts out in South America in a dry season, it sometimes
changes it to a rainy one. The Indians of Paraguay, when their crops are
threatened by drought, set fire to the vast plains with the intention of
producing rain. In Louisiana, heavy rains have been known from time
immemorial to succeed the conflagration of the prairies; and the
inhabitants of Nova Scotia bear testimony to a similar result from the
burning of their forests. Great battles are said to produce rain, and it
is even stated that the spread of manufactures in a particular district
deteriorates the climate of such district, the ascending current
occasioned by the tall chimney of every manufactory tending to produce
rain. In Manchester, for example, it is said to rain six days out of
seven.
[Picture: Decorative picture of person by pool]
[Picture: Decorative picture of pastoral scene with rainbow]