season. A similar sequence is observed on the opposite coasts of the island of Luzon, in the Philippines, and for a similar reason.

But though the sea may assist in the development of rain under suitable circumstances, the presence of water alone will not cause rain. The rainless districts of Peru and Arabia both border on the sea-coast, but no rain falls in either country. In the former the persistent anticyclone which habitually covers that country does not form the ascensional current necessary for rain, though the air is damp enough to deposit very copious dew; in the latter, though the isobars are sometimes curved cyclonically, the rising currents never seem to be sufficiently strong or vapour-laden to produce rain.

Allied to the influence of water in supplying an abundant source of vapour, is the presence of a damp surface, such as a thick forest. The leaves of the trees retain so much moisture that the air is always damper over wood than over earth. Then, as we have just explained in talking about the development of cloud, when an ascensional impulse is propagated over the country, rain will sometimes fall over the woodlands when it would not be precipitated over the cultivated soil. For instance, in the State of Iowa, Dr. Hinrichs has shown that the amount of mean rainfall is very materially influenced by the position of the timber-line, and numerous similar cases have been recorded in other parts of the world.

But observations on this point are very discrepant. In some parts of Germany the influence of forests is said to be enormous; in India the effect appears to be less

marked; while the Swedish meteorologists can find little relation between rainfall and the covering of the earth. The facts are doubtless as they have been described; and the apparent discordance is only another example of the great principle that all meteorological results are the balance of various circumstances. The difference in

humidity over trees or soil will be much less in a cold country like Sweden, than under a blazing Indian sun; and a great deal will depend on whether the principal rainfall is induced by cyclones or secondaries.

MOUNTAIN RAIN.

So far we have treated of water and forest as merely supplying the material for rain; now we must consider a little more in detail how hills and valleys will affect the precipitation of any current. In England, roughly speaking, if a range of hills under about fifteen hundred feet obstructs the prevailing westerly wind, the greatest amount of rain will fall on the east side of the range. This is because, though the moist current is deflected upwards on the west side, the condensed vapour is blown over the top of the hill and falls on the opposite slope. If the range is over fifteen hundred feet, then the rain cannot blow over, and the greatest rain will fall on the west side of the hills. No rule can, however, be laid down except in very general terms, for every hill and every valley has its own local peculiarities in the manner in which it develops rain with different winds. Every country, and every part of each country, must be worked out in detail on the spot.

The amount which will be deposited at different heights will also vary from a number of circumstances. For instance, on the west coast of Scotland, which is constantly exposed to south-west winds, the rainfall on the low western islands is only about forty inches in the year; while along the watershed, which forms the backbone of Scotland, the precipitation exceeds one hundred inches in many places. In Ceylon, to which we have already alluded, the fall on some coast stations does not rise above thirty-four inches, while some of the mountain stations record no less than two hundred and nine inches.

The

difference is readily accounted for when we consider the relative altitudes of the respective mountains and the greatly increased quantity of vapour which an air-current of 90° temperature can carry, compared with one of only 40°. These two latter numbers represent about the mean temperatures of the two countries; and while the watershed of Scotland rarely rises above two thousand five hundred feet, many of the mountains of Ceylon attain an altitude of six thousand feet.

VALLEY RAIN.

It is this property of mountains in developing rain which gives truth to the well-known saying, " Hills draw rain." But there are two sources of rain which are intensified by valleys-the rain of thunderstorms and tidal showers. These we must now consider. In Great Britain it is a common remark that thunder-showers have a tendency to run along the course of rivers. The only class of thunderstorm which does not follow this rule is

that particular kind which occurs in the winter months on the exposed western coast of Scotland and Ireland. These are certainly thunder-squalls, which belong to large cyclones, much developed by mountains. In France and other countries an immense amount of labour has been expended in tracking thunderstorms, as we have already mentioned in a preceding chapter. Though the storms as a whole travel in a north-easterly direction for short distances, the course of rivers is found to exercise a very powerful influence both on their path and still more on their intensity. Forests and hills also modify the development of thunderstorms to a less extent, so that we may conveniently consider them all together.

LOCALIZATION OF HAILSTORMS.

But first we may give an example of the actual facts. Since hail may be considered as the most intense form of a thunderstorm, we have given in Fig. 59 a reduction of a chart illustrating the distribution of hail in the French Department of Loiret.

The river Loire will be readily recognized running across the diagram from right to left, as well as some of its smaller tributaries. A well-known conventional symbol marks the limits of the forest of Orléans, while the small, round points indicate the number of years in which any commune has been attacked by hail during the thirty years 1836-1865. The scale of miles shows what a small area we have to deal with; but see what a difference in the number of hailstorms. In the town of Orléans, on the river Loire, sixteen destructive hailstorms have been

recorded; and at the village of Jangeau, a little higher up the river, twelve serious falls. Yet within a couple of miles of both places, other communes, such as Clery, have not been visited by more than two or three storms. Then observe the influence of the mass of forest to the north of

FIG. 59.-Localization of hail in Loiret.

The points indicate the

number of years in which any commune has been attacked by hail during the thirty years 1836-1865.

the river. All the villages to the west of the forest area have a large number of points, while those inside the forest enjoy almost complete immunity from destructive hailstorms.

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