Fig. 208. A made rain-gauge
Fig. 209. A more complete rain-gauge
As the fall of rain is always measured in inches gauges are made to indicate the equivalent of a cubic inch of rain on the surface of the earth. The simplest form of rain-gauge is a square or circular box or jar with a perfectly flat bottom and perpendicular sides (see [Fig. 207]). If the depth of water in such a gauge be measured after a fall of rain, one can ascertain in inches, or parts of an inch, the amount of rain that has fallen on the surface of the earth. Care must be taken to have the edge of the gauge thin and free from dents, the sides perpendicular and the bottom of the jar perfectly flat, for though in one measurement these irregularities may not make much difference, they would lead to a very decided error in a large number of measurements. Evaporation is also liable in such a gauge to give rise to errors, and extraneous matters are easily introduced. The better rain-gauges are constructed to avoid these contingencies, as far as possible and to depend only on the area of entry for the accuracy of the measurements. This area may be a square, but is usually circular for convenience. The circle must be accurate, and its area is then easily calculated, so that one can estimate the amount of rainfall, however large the receiving vessel may be. The edge of the circle, which may be made of copper, more durable than iron, must be sharp, with an overlapping rim to prevent raindrops from being whirled out of the receiver, and connected by a shoulder to a funnel, which directs the water into the receiver. This may be a glass bottle fitted with a cork to hold the funnel firmly, and prevent leakage between the outside of the funnel and the neck of the bottle (see [Fig. 208]). A more convenient receiver, and one less likely to be broken, is a round tin case of convenient size, with a top fitting accurately under the overlapping edge of the funnel-shaped cover. In this large receiver may be placed a small tin mug, with a lip just under the funnel, for conveniently measuring small quantities of rain, and preventing waste by evaporation. Any overflow from the mug will be caught in the large receiver (see [Fig. 209]). The circle of entry may, of course, be of any size; but one whose diameter is between 4 or 8 inches will be most convenient. Make the circle determine its area by careful measurement, using the following formula: D2 × .7854 = area, each square inch will give cubic inches for area. Take this amount of water and pour it into a glass, marked at the top of the water, and then divide the intervening space between this mark and the bottom into 100 equal parts. This graduated glass will give the rainfall in inches and 100ths of an inch. As an inch glass is somewhat cumbersome, a half-inch glass is usually sent out with a rain-gauge. It may, however, be sometimes convenient to use an ordinary ounce measure, as graduated glass measures, when broken, are not always easily replaced; so that it may be necessary to find the corresponding relation between the cubic inches of receiving area and ounces and drachms. To do this, we will suppose the diameter of the circular top of gauge to be 4.7 inch; this squared = 22.09, multiplied by .7854 = 17.349486, divided by 1.733 (an ounce avoir. = 1.733 c. in.) = 10.011 oz. avoir.
Now if the rainfall is collected daily at a certain time in an ounce measure, the amount may easily be recorded in inches by reference to the accompanying table:
| inch | inch | ||||
| 10 oz. | = | 1.0000 | 1 oz. | = | .1000 |
| 9 " | = | .9000 | 7 dr. | = | .0875 |
| 8 " | = | .8000 | 6 " | = | .0750 |
| 7 " | = | .7000 | 5 " | = | .0625 |
| 6 " | = | .6000 | 4 " | = | .0500 |
| 5 " | = | .5000 | 3 " | = | .0375 |
| 4 " | = | .4000 | 2 " | = | .0250 |
| 3 " | = | .3000 | 1 " | = | .0125 |
| 2 " | = | .2000 |
A similar calculation can be made and table prepared for any larger circle of entry by the same method.
The amount of rainfall in any country is a matter of great importance to that country, and, like the rise of the Nile in Egypt, it indicates the coming state of the crops. If we have too small a rainfall, drought and withered crops follow, and if we get too great a fall of rain, drowned out crops, and disastrous floods occur, so you see how necessary it is that those people who are elected to look after the welfare of a nation, should keep posted on matters of rainfall in all its phases. In India, China and some other parts of the world the question of rainfall is one of life and death to the people, and most of the great famines of the past have been due to the small rainfall. Hundreds of thousands of people used to perish by famine and disease year after year. Much of this danger from shortage of rain has happily been avoided in India by the efforts of the British government, which has inaugurated and carried out great schemes of irrigation and artificial waterways to prevent the recurrence of famine from drought. Our own government also is expending large sums of money on irrigation plans now being executed in Arizona, Texas, Colorado and other states, which will render immense territories fit for cultivation, which would otherwise have remained barren and of no use. The matter of rainfall is of the highest importance to a nation and to the men and beasts inhabiting it.
"Will it rain to-day?" is a question frequently asked, as regards the weather, showing how important the subject is, and while I am talking on it, it may not be amiss to make a few remarks regarding the formation and distribution of rain, as formulated by learned meteorologists. We are told that the two great causes of rain are the sun and the ocean—the latter, of course, includes the great lakes and rivers—and since these two factors may be taken as constant, it follows that the rainfall over the earth as a whole will always be constant, while the local variations will be due to local conditions. The rain which falls on this continent is drawn up by the sun from the various sources, but the conditions which cause its precipitation may be said to be local. To your imagination may be left the tracing of the journey of the rain drops back to the ocean again. The starting points in considering the causes of rain are, therefore, heat and moisture. From the surface of land and water moisture is continually evaporating into the atmosphere, and the higher the temperature of the air the more watery particles it can hold. If any reduction in the temperature of this saturated air should take place, the vapour becomes visible as fog, mist, or cloud, and it is from this vapour that the rain drops are formed. Recent research says that these watery particles require minute dust atoms as nuclei before they can form, and it has been estimated, by experiment, that there are one thousand millions of them in a cubic foot of saturated air, though their total weight amounts to only 3 grains. Accepting these figures, the mathematically inclined may be told that it would require a cloud three miles thick to produce one inch of rainfall. But before these watery particles can fall to the earth as rain, they must first form into rain drops, and the question arises, how are rain drops formed?