I may now draw some inferences from the above observations. In the first place, it is probable that the annual coast rainfall of Bougainville Straits and that of the eastern end of the Solomon Group are much about the same, viz., about 150 inches: the chief difference between the two regions being, that in the former region, there are a greater number of rainy days and fewer heavy falls. The heavy falls, when they do occur, are not easily forgotten; thus, at Treasury we measured, in July, 1884, 11 inches of rain in 10 successive hours; but the daily record was only 8·09 inches, since the rain began in the evening of one day and lasted well into the following morning.

During the heavy rainfalls in these regions the streams swell in an astonishingly quick manner. Rivulets become turbid streams, the whole hill-slope discharges a continuous sheet of water, and the water rushes down the permanent stream-courses with the roar of a mountain-torrent. Large blocks of stone are swept some distance along the lower courses of the streams; and the trunks of trees are carried by each successive flood further and further towards the mouth of the stream.

It should be now remarked that the average rainfall for the year, which I have estimated from observations made in different parts of the Solomon Group at about 150 inches, only applies to the coast. It is probable that this estimate is generally applicable to the coasts of these islands, except on the lee sides of the loftier islands.[494]

[494] By the lee sides, I mean those sheltered from the prevailing S.E. trade.

This brings me to the question of the rainfall in the higher regions. The rainfall will increase with elevation until a certain height is reached, where the clouds attain their maximum density; at such a level the greatest rainfall will occur. I learn from an interesting paper by Mr. Bateman on this subject,[495] that it may be inferred that in the Lake District of England the greatest rainfall occurs at an elevation of 2,000 feet, which is the level of maximum cloud density. In India, an elevation of 4,500 feet represents the level at which the greatest rainfall occurs. In the Solomon Islands, a greater height will have to be attained before the level of maximum cloud density or that of the greatest rainfall will be attained. Probably I shall not greatly err if I assume it to be between 5,000 and 6,000 feet. I have already observed that the south-east trade, subject to its usual variations, is the prevailing wind in the eastern part of the group for nearly two-thirds of the year. Coming laden with its watery burden, it first strikes the eastern slopes of St. Christoval; but although the higher regions of this island must cause the rain-clouds to precipitate a large amount of their moisture, the higher peaks do not rise in sufficient mass to a height that would receive the greatest rainfall, the extreme height being 4,100 feet. The rain-clouds, with the bulk of their moisture, would therefore be driven over the higher regions of this island, and would deposit the greater part of their burden on the higher slopes of the mountainous eastern portion of Guadalcanar. Since this island, in its eastern portion, rises in mass to a height of some 5,000 feet and attains a maximum elevation of 8,000 feet, it does not seem probable that, during the prevalence of the trade for nearly two-thirds of the year, a considerable quantity of rain would be deposited on the western side of the island; and, that such is the case, is shown in the fact that the dense forest-growth that clothes the steep eastern and southern slopes of the island gives place, on the lee or west side of the mountains, to a vegetation which gives to the western portion of Guadalcanar, when viewed from seaward, the appearance of a savannah or a prairie.

[495] Journal of the Victoria Institute. Vol. XV. No. 59.

The lofty mountain-masses of the east end of Guadalcanar, which forms one of the finest specimens of coast-scenery in the world, are usually enveloped in rain-clouds at their summits. But occasionally one of the peaks is visible above the thick cloud-covering, marking by its elevation, as it were, the line of greatest rainfall lying below. In the same manner the high peaks at the east end of Bougainville, which have an elevation of between 7,000 and 8,000 feet, may be seen occasionally to project above the rain-clouds; but there is, probably, a smaller quantity of rain deposited on the higher slopes of this island than on those of Guadalcanar, because the mountains are more isolated, possess for the most part the tapering volcanic profile, and do not rise “en masse,” as in the case of the high lands of Guadalcanar. The greatest rainfall in the Solomon Group takes place on the steep southern and eastern slopes of this island of Guadalcanar. Huge mountain-masses appear to rise directly from the sea to a height of some 5,000 feet, ultimately attaining a height of 8,000 feet. The fall there must be tremendous, especially when, as is frequently the case, the land of St. Christoval does not interpose itself in the path of the moisture-laden trade-wind. Then, loaded with vapours after its passage across a wide expanse of ocean, and with but a thin tract of intervening lowland to rob it of its moisture, the trade strikes at once upon the precipitous mountain-slopes as against some Cyclopean rampart. There is no ravine or breach in the mountain-mass to ease the tension. There, on those mountain-slopes, a terrific precipitation must occur, which, if the annual rainfall of the coast is 150 inches, will here be three or four times that amount. This is no exaggerated language, but is the opinion I have formed, after having carefully considered the physical geography of these regions.

The subjoined rainfalls of a few localities in other parts of the world may be interesting to compare with that of the Solomon Group:[496]

[496] Somerville’s “Physical Geography,” 7th edit. pp. 331-334.

England32inches.
Singapore97„
Atlantic Doldrums225„
Western Ghats302„
Cherraponjee10„
Solomon Islands.
(a.) at the coast150„
(b.) on the higher slopes of Guadalcanar400to 500 inches probably.