From Dr. Buchan’s statistics it is found that the annual rainfall at Mahabaleshwar is 263 inches; at Sandoway 214; and at Cherra-pungi 472 inches, the largest known rainfall anywhere on the globe. Over a large part of the Highlands of Scotland more than 80 inches fall annually, while in some of the best agricultural districts there it does not exceed 30 inches.
Of all meteorological phenomena, rainfall is the most variable and uncertain. Symons gives as tentative results from twenty years’ observations in London—(1) In winter, the nights are wetter than the days; (2) in spring and autumn, there is not much difference; (3) in summer, nearly half as much again by day as by night.
The wearisomeness of statistics may be here relieved by a short consideration of the splash of a drop of rain. Watching the drop-splashes on a rainy day in the outskirts of the city, when unable to get out, I brought to my recollection the marvellous series of experiments made by Professor A. M. Worthington in connection with these phenomena. Of course, I could not see to proper advantage the formation of the splashes, as the heavy raindrops fell into these tiny lakes on the quiet road. There is not the effect of the huge thunder-drops in a stream or pool. The building up of the bubbles is not here perfect, for the domes fail to close, nor are the emergent columns visible to the naked eye. It is a pity; for R. L. Stevenson once wrote of them in his delightful “Inland Voyage,” when he canoed in the Belgian canals, as thrown up by the rain into “an infinity of little crystal fountains.”
Beautiful is this effect if one is under shelter, every dome seeming quite different in contour and individuality from all the rest. But terrible is it when out fishing on Loch Earn, even with the good-humoured old Admiral, when the heavy thunder-drops splash up the crystal water, and one gets soaked to the skin, sportsman-like despising an umbrella.
There is, however, a scientific interest about the splash of a drop. The phenomenon can be best seen indoors by letting a drop of ink fall upon the surface of pure water in a tumbler, which stands on white paper. It is an exquisitely regulated phenomenon, which very ideally illustrates some of the fundamental properties of fluids.
When a drop of milk is let fall upon water coloured with aniline dye, the centre column of the splash is nearly cylindrical, and breaks up into drops before or during its subsequent descent into the liquid. As it disappears below the surface, the outward and downward flow causes a hollow to be again formed, up the sides of which a ring of milk is carried; while the remainder descends to be torn a second time into a beautiful vortex ring. This shell or dome is a characteristic of all splashes made by large drops falling from a considerable height, and is extremely pretty. Sometimes the dome closes permanently over the imprisoned air, and forms a large bubble floating upon the water. The most successful experiments, however, have been carried through by means of instantaneous photography, with the aid of a Leyden-jar spark, whose duration was less than the ten-millionth of a second. But the simple experiments, without the use of the apparatus, will while away a few hours on a rainy afternoon, when condemned to the penance of keeping within doors.
CHAPTER XXXI
THE METEOROLOGY OF BEN NEVIS
Several large and very important volumes of the Royal Society of Edinburgh are devoted to statistics connected with the meteorology of Ben Nevis. Most of the abstracts have been arranged by Dr. Buchan; while Messrs. Buchanan, Omond, and Rankine have taken a fair share of the work.