816 REPORT— 1900. 



movement as compared with that of other tropical countries, and to form some 

 conclusion as to the permanence, or the reverse, of present conditions. 



Among historical reasons for the smallness of the existing trade are (1) the 

 attraction exercised during the age of great discoveries by America and the East 

 and the consequent neglect of Africa ; (2) the political condition of the African 

 peoples ; (3) tbe effects of the slave trade ; whil-' geographical causes are found 

 in (1) the massive form of the continent and consequent absence of natural means 

 of communication ; (2) the unhealthiness of the coastlands. That many of these 

 causes are not necessarily permanent is shown by a comparison with Brazil, which 

 affords a close parallel with Tropical Africa in many respects. This shows that, 

 given natural resources capable of supporting an increased export trade, the com- 

 mercial future of Tropical Africa need not be hopeless. 



The resources of a new country may be classed as (1) exhaustible, principally 

 minerals ; (2) permanent, chiefly animal and vegetable products, the second group 

 being the more important. It may be again subdivided into (1) jungle products, 

 which, though not necessarily exhaustible, are likely to siiH'er diminution ; (2) 

 cultivated products. The former may, under cultivation, be transferred to the 

 latter sub-group, which is the most important of all. In Brazil, e.g., the vast 

 preponderance of the exports is made up by the four products coffee, cacao, 

 tobacco and cotton. Which, with rubber, maiie up the principal resources of the 

 countrv. In Tropical Africa jungle products, principally rubber and palm-oil 

 and kernels (total annual value over 4,000,000/.), are at present those on which 

 the export trade mainly depends. A period of development of plantation pro- 

 ducts has, however, set in, and coffee, cacao, cotton, tea, &c., have been grown 

 with success in various parts. The chief difficulties to be encountered arise from 

 (1) want of means of transport ; (2) scarcity of labour ; but these are now in a 

 fair way to be overcome. The modern tendency for each country to depend for 

 tropical produce largely on its own colonies must favour the commercial develop- 

 ment of Africa, while the comparatively low population of Africa per square mile 

 renders it probable that it will in the future play an important part in providing a 

 food supply for the more thickly peopled continents. 



3. On Snow Ripples. 

 By Yaughan Cornish, M.Sc, F.C.S., F.R.G.S. 



These observations, made in Scotland last winter, are preliminary to a general 

 investigation of the surface forms of snow, which the author proposes to continue 

 in a colder climate during the coming winter. The investigation is undertaken in 

 connection with the author's research upon terrestrial waves and wave-like sur- 

 faces. 



The general conditions during the following observations are : ground already 

 covered with snow, temperature a little below the freezing point. 



Case I. — Snow falling sparsely. In absence of wind the surface was uneven, 

 owing to clinging together of flakes. In a light breeze there was a notable 

 tendency for the prominent parts to arrange themselves transversely in ridges, the 

 distance from ridge to ridge not more than one inch. When the breeze freshened 

 these became regular ripples, with a smoothed surface of closer texture. One set of 

 measurements gave the distance between successive ridges, 1125, 1'225,0'85, 1"05,. 

 and I'OO inch. Their amplitude was approximatslj -05 inch, which gives a ratio 

 Length : Height = 21 approximately. The steep face of these ripples is on the 

 windward side, whereas in sand ripples and water waves the steep face is on the 

 sheltered side. The normal movement is downwind, the most noticeable feature 

 of the pi'ocess being the retreat of the steep weather face, consequent upon the 

 abrasion of its surface. For occasional short intervals, however, during lulls and 

 during moments of heavier snowfall, the ripple.5 rush upwind, owing to the sudden 

 deposit of snow upon each weather face. 



Case II. — Frosh breeze without snowfall, blowing upon uncompacted snow. 

 The surface was beautifully covered by ripples of '6 inches to 15 inches from ridge 



