CHAPTER XX.
416. _What is rain?_
Rain is the _vapour of the clouds_ which, being condensed by a fall of temperature, forms drops of water that descend to the earth.
It is the _return to the earth_ in the form of _water_, of the moisture _absorbed by the air_ in the form of _vapour_.
417. _Does rain ever occur without clouds?_
It sometimes, but rarely happens, that a sudden transition from warmth to cold will _precipitate the moisture of the air_, without the formation of _visible clouds_.
[Verse: "Canst thou lift up thy voice to the clouds, that abundance of waters may cover thee?"--JOB XXXVIII.]
418. _Why are drops of rain sometimes large and at other times small?_
Because the drops, in falling, _meet and unite_, and also gather _moisture_ in their descent. The greater the height from which a rain drop has descended, _the larger it is_, provided that its whole course lay through a _rainy atmosphere_.
The size of the drops is also influenced by the _amount of moisture in the atmosphere_, the _degree of cold_, and the _rapidity_ of the _change of temperature_, by which the drops are produced.
419. _In what seasons of the year are rains most prevalent?_
Throughout _Central Europe_ rains are most prevalent in _summer_, but in _Southern Europe_ the preponderance is on the side of _winter rains_.
420. _In what months of the year does it rain most frequently in this country?_
It rains more frequently _from September to March_, than from _March to September_; but the _heaviest rains_ occur from _March to September_.
421. _Why are there more rainy days from September to March?_
Because the temperature of the air is more frequently lowered to that degree which _precipitates its vapours_.
_Months in the order of their comparative wetness_:--1. October. 2. February. 3. July. 4. September. 5. January. 6. December.
_Months in the order of their comparative dryness_:--1. March. 2. January. 3. May. 4. August. 5. April. 6. November.
422. _In what part of the world does the greatest quantity of rain fall?_
The greatest _quantity_ of rain falls near the _equator_, and the amount _decreases towards the poles_.
[Verse: "Who can number the clouds in wisdom? or who can stay the bottles of heaven."--JOB XXXVIII.]
423. _In what part of the world do the heaviest rains occur?_
The _heaviest_ rains occur in the _tropics_, during the hot season. The drops of rain in the tropical regions are so large, and the force with which they descend so great, that their splash upon the skin causes a _smarting sensation._
424. _In what parts of the world do the least rains occur?_
There are some parts of the earth which are _rainless_, such as Egypt, the desert of Sahara, the table lands of Persia and Montgolia, the rocky flat of Arabia Petræ, &c.
425. _How many rainy days are there in a year?_
The frequency of rainy days is greatest in countries near the sea, and their number decreases the further we journey from the sea-border towards the inland. In England it rains on an average 152 to 155 days in the year.
426. _In what part of England does the greatest amount of rain fall?_
In the town of _Keswick_, in Cumberland, where 63 inches of rain fall in a year; Kendal, in Westmoreland, 58 inches; Liverpool, 34 inches; Dublin, 25 inches; Lincoln, 24 inches; London, 21 inches.
427. _Why do the heaviest rains occur at the tropics?_
Because the _hot air_ absorbs a large amount of vapour, and rises into the higher regions of the atmosphere, where the vapours are _suddenly condensed into heavy rains_, by cold currents from the poles.
428. _Why does the greatest quantity of rain fall at the equator?_
Because the _hot air_ absorbs a large amount of vapour, and as the atmosphere is usually calm, there is an absence of currents, by which the saturated air would be removed. In this, which is called "_the Region of Calms_," rain falls almost daily.
429. _Why are some parts of the earth rainless?_
Because, being situated in tropical or torrid latitudes, and at a distance from the ocean, the atmosphere above them is always in a _dry state_.
[Verse: "Thou, O God, didst send a plentiful rain, whereby thou didst confirm thine inheritance, when it was weary."--PSALM LXVIII.]
430. _When is air said to be saturated with vapour?_
When it cannot take up _a larger quantity_ than that which it already holds.
When common salt is dissolved in water, until the water can take up no more, the water is then said to be _saturated with salt_.
431. _What proportion of water is air capable of sustaining in the form of vapour?_
The amount of water held in suspension by the air averages the following proportion: one thousand _cubic feet of air_ contain as much vapour as, were it condensed to water, would yield about _two fifths of a pint_.
But _one thousand cubic feet of air_ are capable of holding _half-a-pint of water_; and this may be regarded as the _point of saturation_.
Thus, in a room ten feet square and ten feet high, the air, _at the point of saturation_, would hold in the form of vapour, _half-a-pint of water_. It must not be forgotten, however, that the point of saturation necessarily varies with the _temperature of the air_.
432. _Why are cloudy days and nights not always wet?_
Because the air has not reached the state of _saturation_.
433. _Why does rain purify the air?_
Because it produces motion in the particles of the air, by which they are _intermixed_. And it precipitates noxious _vapours_, and cleanses the face of the earth from _unhealthy accumulations_.
434. _Why are mountainous localities more rainy than flat ones?_
Because the mountains _attract the clouds_; and because the clouds that are flying low are borne against the sides of the mountains and directed upwards, where they meet with _cold currents of air_.
435. _Why does more rain fall by night than by day?_
Because by night the temperature of the air, heated during the day, falls to that degree which condenses _its vapours into rain_.
[Verse: "As the hart panteth after the water brooks, so panteth my soul after thee O God."--PSALM XLII.]
436. _Why do bunches of dried sea-weed indicate the probability of coming rain?_
Because they readily imbibe moisture, and when they become soft and damp they show that the air is _approaching the point of saturation_.
437. _Why does the weather-toy, called the "weather-cock," foretell the probability of rain?_
Because it is made with a piece of cat-gut which swells with moisture, and as it swells, _shrinks_. The cat-gut is so applied that when it _shrinks_, it turns a rod which sends the _man_ out of the house, and when it _dries_ it sends the _woman out_. Therefore, when the _man_ appears, it is a sign of _wet_, and when the _woman_ appears it is a sign of _dry weather_.
There is another toy, called the Capuchin, which is made upon the same principle. The figure lifts a hood over its head when wet is approaching, and takes it off when the weather is becoming dry. In this case, a piece of cat-gut is also employed. Various weather-toys may be made upon this principle--among others, a little umbrella, which will open on the approach of wet, and close on the return of fine weather.
A gentleman once made a wooden horse, which he declared should of itself walk across a room, without machinery of any kind. The assertion was discredited; but the horse was placed in a room close to the wall on one side. The room was locked, and otherwise fastened, so that no one could interfere with the experiment. After a time the door was opened, and it was found that the horse had actually crossed the floor, and stood on the opposite side. The horse was made from wood of a peculiar kind, liable to great expansion in wet weather, and cut in a manner to produce the greatest elongation. The fore hoofs were so made that where they were set they would remain, so that the contracting parts should draw up from behind. It is easy to understand how, in this way, the wooden horse crossed the apartment.
438. _Why does ladies' hair drop out of curl upon the approach of damp weather?_
Because the hair _absorbs moisture_, which causes its spirals to relax and unfold.
439. _Why is it said in mountainous countries that rain is coming, because the mountains are "putting their night-caps on?"_
Because the clouds descend when they are _heavy with vapour_, and being attracted to the mountain tops they are said to "_cap the mountains."_
[Verse: "Hast thou entered into the treasures of the snow; or hast thou seen the treasures of the hail."--JOB XXXVIII.]