INSTRUMENTS USED FOR MEASURING THE RAINFALL.
The instruments in use for measuring the quantity of rain which falls on a
given spot are of very simple construction. Perhaps the simplest is:--
=110. Howard's Rain-Gauge.=--It consists of a copper funnel, a stout glass
or stone bottle, and a measuring glass. The bottle is to be placed upon
the ground, with the funnel resting on its neck. A brass band or cylinder
fixed upon the outer surface of the funnel envelops the neck of the
bottle, and the pipe of the funnel extends nearly to the bottom of the
bottle; so that loss by evaporation is avoided as much as possible. The
receiving space of the funnel is formed by a brass ring, five inches in
diameter, very accurately turned. The measuring vessel enables the
observer to note the rainfall in inches, tenths, and hundredths of an
inch.
[Illustration: Fig. 84.]
=111. Glaisher's Rain-Gauge.=--The rain-gauge designed by Mr. Glaisher,
the well-known meteorologist, and used by most observers of the present
day, is arranged for the reception of the water which falls upon its
receiving surface only, and for the prevention of loss by evaporation. The
rain is first collected in a funnel, _B_, (fig. 84,) the receiving surface
of which is turned in a lathe. The conical surface of the funnel slopes to
the pipe, _E_, at an angle of 60 deg. from the horizontal receiving surface.
The tube, _E_, is of small aperture, and is bent up, in order to retain
the last few drops of rain, so that the only opening for the escape of
vapour may be closed as long as possible. The funnel, _B_, fits upon the
cylinder, _A_, tightly in the groove, _D_. A copper can is placed inside
the cylinder, _A_, to receive the rain from the funnel. Once or twice a
day, or after a shower, this can should be taken out, and the water
measured in the glass measure, _C_, which is graduated to hundredths of an
inch, according to the calculated quantity of water, determined by the
area of the receiving space. In use, this gauge should be partly sunk in
the ground, so that the top may be about five inches above it. Thus
situated, there will be little or no evaporation from it during any month
of the year; and the readings need not be taken daily, although desirable.
=112. Rain-Gauge with Float.=--In this construction the graduated glass
measure is dispensed with. The cylinder of the gauge is made less in
diameter than the funnel, and a hollow, very flattened spheroid of copper
forming a float, and carrying a vertical graduated boxwood scale which
moves through the orifice of the funnel, is placed in it. As the rain
accumulates the float rises, and the amount of rain in the gauge is read
upon the scale from the top of the gauge, a bar, having a hole at the
centre for the passage of the scale, being fixed diametrically across the
receiving space of the funnel. The gauge is provided at the bottom with a
brass cock, by which the water may be allowed to flow out of it whenever
necessary.
This form of gauge is not very suitable for the measurement of small
quantities; but is admirably adapted for localities where the rainfall is
excessive.
[Illustration: Fig. 85.]
=113. Rain-Gauge with Side-Tube.=--This instrument, as represented in fig.
85, is a cylindrical vessel, mounted on a base shaped as a frustum of a
cone. This base may be filled with sand or gravel to make the instrument
stable, so that when placed upon a lawn or in a garden it may have an
ornamental appearance. The funnel for collecting the rain is larger in
diameter than the cylinder. Parallel to the cylinder, and communicating
with the lowest part of the interior and extending to its top, is a
graduated glass tube, open at both ends. The rain collected will rise as
high in this tube as in the cylinder, and its amount can therefore be read
off without any trouble. The gauge is emptied by the brass tap at the
bottom of the cylinder.
=114. Admiral FitzRoy's Rain-Gauge.=--A form of rain-gauge, very well
adapted for expeditious observation at any time, has been designed by
Admiral FitzRoy, and extensively employed by his observers. It is
cylindrical in shape, with the funnel let into the top; and the rainfall
is collected in an inner and much smaller cylinder, so that a small fall
is represented by a considerable depth of water in the gauge. The amount
of rain which has fallen is ascertained by a dipping tube, similar in
principle to the dipping syphon used by gaugers for taking out specimens
of wines or spirits from casks by simply removing the bung. A short,
vertical, tubular opening provided with a cap, which is attached to the
instrument by a chain that it may not be lost, is formed in the funnel.
The measuring tube, which has a small hole at each end, should be placed
upright in the gauge; then the thumb should be pressed over the upper
aperture, while the tube is lifted gently out, holding in the lower part a
quantity of water representing the depth of the rain in the gauge, the
upper edge of which is at the mark to be read off. The glass tube is
graduated to inches and tenths; hundredths of an inch can be readily
estimated by the eye. The marks are fixed by actual trial with a standard
gauge, and are artificial, not true, inches.
=115. Self-Registering Rain-Gauge.=--The rain-gauge can be combined with
clock-work and other mechanism so as to be self-recording of the amount of
rain, the time, and duration of its fall. For the details of construction
the reader is referred to the next chapter, where he will find the
instrument described in connection with Osler's anemometer, as the
"pluviometer." To observe and duly record the times of commencement and
termination of rain is very desirable. Scarcely any observer can attempt
to do this even approximately from personal observation. Hence the want of
a cheap and simple self-recording rain-gauge is much felt, the present
construction being too expensive for all but a few individuals.
In 1862, Mr. R. Strachan estimated the duration and amount of rain in
London (Gray's Inn Road) as follows:--
+-------------------------------------------------------------------+
| MONTHS. |INCHES.|DAYS.|HOURS.|| MONTHS. |INCHES.|DAYS.|HOURS.|
|-----------+-------+-----+------++------------+-------+-----+------|
|January. | 1.86 | 19 | 88 ||July. | 2.27 | 17 | 68 |
|February. | 0.37 | 9 | 25 ||August. | 2.45 | 12 | 72 |
|March. | 3.40 | 22 | 130 ||September. | 1.70 | 12 | 55 |
|April. | 2.34 | 14 | 80 ||October. | 3.23 | 21 | 94 |
|May. | 3.04 | 16 | 90 ||November. | 1.12 | 10 | 53 |
|June. | 2.45 | 20 | 83 ||December. | 1.44 | 17 | 66 |
+-------------------------------------------------------------------+
"During the year 1862, the rainfall amounted to 25.67 inches. Rain fell on
179 days, that is, on nearly every other day. The hours of rain were
estimated at 904; therefore, if the rain had fallen continuously, it would
have lasted nearly 38 days and nights."[10] The value of similar estimates
of the rainfall by numerous observers would be very great to meteorology.
=116. The principle of measurement= in all these gauges is the relation
existing between the areas of the collecting and receiving surfaces; that
is, between the area of the funnel into which the rain falls, and the area
of the cylinder which receives it. In Howard's and Glaisher's gauges, this
cylinder is virtually the measuring glass itself; in the others, above
described, the measuring scales show the same depth of water as in the
cylinder of the gauge.
The cylinder being of less diameter than the funnel, and receiving all the
rain collected by the funnel, it follows that its contents will have an
increased depth. Now equal cylindrical volumes, having different
diameters, are to each other in length inversely as the squares of the
diameters. Hence, if the funnel be 9 inches and the cylinder 3 inches in
diameter, a fall of 1 inch of rain will be represented in the gauge by 9
inches; for 3 squared : 9 squared :: 1 : _x_ = 9. In this case, therefore, a length of
nine inches of the measuring glass, tube, or scale, would represent an
inch of rainfall, and be divided into tenths and hundredths of the
artificial inch.
=117. Position for Rain-Gauge, &c.=--Rain gauges should be placed on the
ground, in any position exposed to a free fall of rain, snow, or hail,
where neither walls, buildings, nor trees shelter or cause eddies of wind.
They should be supported by a frame, or other means, to prevent them being
blown down by the wind, but so that they can be readily emptied.
During snow or frost, the gauge must be watched, and its contents melted
by placing it in a warm room, either when the amount is to be measured, or
the funnel is filled up with snow. A tin vessel of equal area to the
funnel may at such times be useful as a substitute.
Rain gauges are constructed of metal, usually copper, which, besides being
readily workable, is little affected by atmospheric influences. If made of
iron or zinc, they should be well japanned; if of copper, this is not so
essential. The capacity of a gauge should be sufficient to contain at
least the probable maximum fall of rain in a day at the locality. Those
required for rainy districts must be of large size.
=118. Causes of Rain.=--When the invisible vapour which is diffused in the
atmosphere becomes sufficiently cooled, it appears visible as mist or
cloud, and a further reduction of temperature causes its precipitation as
rain, hail, or snow. The cooling of the higher regions of the atmosphere
is doubtless the chief cause of this condensation; but the property which
aqueous vapour possesses of radiating heat may also contribute to the
result. Moreover, the law which regulates the amount of vapour which air
at any particular temperature can sustain in a transparent state,
determines that when two bodies of air at different temperatures,
saturated with vapour, intermix, some moisture must be rendered visible;
and hence, it is not only possible, but highly probable, that rain may
result from the conflict of different winds. Let us imagine two cubic
yards of air, both saturated with moisture, but having the respective
temperatures of 50 and 70 degrees, to come into contact. There will be a
tendency to equalize the temperature to a mean, which is 60 deg.; and during
this process, some of the vapour will be condensed.
For in the air at 50 deg. there is 110.7 grains of vapour[11]
and " 70 " 216.0 "
------
Total amount of vapour 326.7 "
But two cubic yards of air at 60 deg. can only sustain 313.2 "
------
Hence there will be deposited 13.5 " of rain.
======
It may be conceded, therefore, that when a warm and moist current of air
encounters a body of cold air which may not be extremely dry, the mixture
is unable to retain the whole of the vapour in an invisible state; so that
the excess becomes visible as mist or fog, and, when the temperature has
become sufficiently lowered, rain. The British Isles are more or less
enveloped in fog, or mist, at the commencement of easterly winds, which,
with a sudden change of wind, is exhibited even in summer; while the
south-westerly winds, warm, and arriving from the ocean, deposit large
quantities of rain by the cooling effect of the land, colder by reason of
its latitude. When rain occurs with a northerly wind, it is probably due
to the deposition from an upper south-westerly current, often apparently
proved by the movements of the upper clouds.
=119. Laws of Rain-fall.=--Tropical countries have a dry and a wet season
during the year: _dry_, when the sun is at the opposite side of the
equator; _wet_, when the sun is overhead. With reference to the British
Isles, the statistics collected by Mr. G. J. Symons indicate that: 1st.
The stations of least rain are inland, or on the east or south-east
coasts; the stations of greatest rain are on the western coasts. 2nd. The
rain-fall is very large in the vicinity of mountain chains or groups,
unless the station happens to be some miles to the north-eastward.
It may be well to illustrate these remarks by quoting[12] the average fall
at a few places, grouping them as--
Westerly.
Inches.
Bodmin 43
Bolton (Lancashire) 44
Coniston (Windermere) 71
Seathwaite 127
Torosay (I. of Mull) 75
Killaloe (Limerick) 38
Central.
Inches.
Enfield 23
Epping 23
Derby 24
York 22
Stirling 39
Perth 29
Easterly.
Inches.
Witham (Essex) 21
Patrington (Hull) 21
Sunderland 17
Inveresk (Edinburgh) 25
Pittenweem (Fife) 24
Dublin 22
Mr. Green, the celebrated aeronaut, has asserted from his experience,
"that whenever a fall of rain happens, and the sky is entirely overcast,
there will invariably be found to exist another stratum of cloud at a
certain elevation above the former;" and the recent scientific balloon
ascents by Mr. Glaisher have tended to confirm this theory. Mr. Glaisher
says, "It would seem to be an established fact, that whenever rain is
falling from an overcast sky, there is a second stratum above." "It would
also seem that when the sky is overcast without rain, that there is no
stratum of cloud above, but that the sun is shining on the upper surface.
In every instance in which I have been up under these circumstances, I
have found such to be the case, agreeing in this respect also with Mr.
Green's observations."
The amount of rain collected in a gauge placed near the surface of the
earth is larger than in any gauge placed above it; and the higher the
gauge is placed, the less water is collected. Mr. Glaisher contends that
his balloon experiments corroborate this law.
=120. Utility of Statistics of Rain-fall.=--The utility of knowing the
rain-fall of any locality is sufficiently obvious, and little need be said
upon the subject. The rain-gauge should be in the hands of every gardener
and farmer. In the management of out-door plants and crops, as well as in
the construction of cisterns and tanks for the supply of water, a rain
gauge is a valuable assistant. By its use, the gardener will be guided in
judging how far the supply of moisture to the earth is needed; and he will
also see how beneficial is even a hasty shower to growing plants, when he
considers that a fall of rain measuring the tenth of an inch in depth,
corresponds to the deposit of about forty hogsheads per acre. The study of
the rain-fall of a country is of considerable interest to agriculturists.
The health and increase of domestic animals, the development of the
productions of the land, as well as the daily labours of the farmer, are
dependent upon the excess or deficiency of rain. "It must be a subject of
great satisfaction and confidence to the husbandman to know at the
beginning of a summer, by the certain evidence of meteorological results
on record, that the season, in the ordinary course of things, may be
expected to be a dry and warm one; or to find, in a certain period of it,
that the average quantity of rain to be expected for the month has fallen.
On the other hand, when there is reason, from the same source of
information, to expect much rain, the man who has courage to begin his
operations under an unfavourable sky, but with good ground to conclude,
from the state of his instruments and his collateral knowledge, that a
fair interval is approaching, may often be profiting by his observations;
while his cautious neighbour who waited 'for the weather to settle' may
find that he has let the opportunity go by. This superiority, however, is
attainable by a very moderate share of application to the subject; and by
the keeping of a plain diary of the barometer and rain-gauge, with the
hygrometer and vane, under his daily notice."[13] The statistics of
rain-fall are not only valuable and interesting in a meteorological point
of view, and for agricultural purposes, but are also highly important in
connection with sanitary arrangements for towns, and engineering
operations. This is especially evident to the hydraulic engineer. As rain
is an important source of water-supply to rivers, canals, and reservoirs,
it is evident that a knowledge of the probable fall for any season or
month, at a given place, as furnished by averages of the observations of
former years, will be the data upon which the engineer will base his plans
for providing for floods or droughts; while the measurement of the actual
quantity which has just fallen, as gathered from the indications of a
series of gauges, will suggest to him the precautions to adopt either to
economise or conduct away the in-pouring waters.
"When a canal is conducted across an undulating country, its course is
necessarily governed by the accidents of the ground, and it alternately
rises and falls. In this case, rising by a succession of levels, it
necessarily arrives at a certain highest level, which is called by
engineers the _summit level_. From this it again descends by a
corresponding series of levels. Now, it is evident that, supposing the
locks to be all equal in magnitude, the ascent of a vessel will require
the descent of as much water from the summit to the lowest level as would
fill a single lock; for this quantity of water must be discharged from
each lock of the series when the vessel passes through it.
"The same may be said of the process by which the vessel descends along
the series of locks on the other side of the summit. It appears,
therefore, that a supply of water must always be maintained on the summit
level sufficient to fill a single lock twice for each vessel which crosses
the summit.
"It happens, fortunately, that by the laws of natural evaporation, rain is
precipitated in greater quantities on elevated summits than on the
intermediate valleys, so that the moving power, in this case, accommodates
itself to the exigencies of intercommunication."--_Dr. Lardner's "Handbook
of Natural Philosophy."_
=121. New Form of Rain-Gauge.=--Since the foregoing pages were in type, a
modification of Howard's rain-gauge has been arranged by Mr. Symons, which
is compact in design, convenient in use, and low in price. It combines the
advantages of most gauges; having solidity, and facility of measurement.
The bottle is placed in a tin case, to the bottom of which are attached
stout spikes, which, when forced into the earth, prevent its being upset
either by wind or accident. The bottle being transparent, and slits made
in the case, the fall of rain is seen at a glance, or with a race-glass,
from a window. The funnel being attached to the cover of the case is
thereby kept strictly horizontal, and the depth of rain can be accurately
measured by lifting the bottle from its case and emptying it into a
graduated glass jar.
The funnel of this gauge is a very deep cone, to prevent the rain drops
outsplashing. When properly placed, the receiving surface will be twelve
inches above the ground, which experience has shown to be the most
advantageous height.