G.—ENGINEERING. 191 
possible to obtain what is often a sufficiently close approximation to 
the iun off, but even rainfall records are not generally at hand where 
they are most required, and even in a districh where such records are 
available, they are usually confined to easily accessible points, and are 
seldom extended to the higher levels of a catchment area where the 
rainfall is greatest. Even throughout the United Kingdom our know- 
ledge of the rainfall at elevations exceeding 500 feet is not satisfactory, 
and little definite is known concerning that at elevations exceeding 
1,000 feet. 
In this country evaporation may account for between 20 and 50 per 
cent. of the annual rainfall, depending on the physical characteristics 
of the site, its exposure, mean temperature, and the type of surface 
covering. In some countries evaporation may account for anything 
up to 100 per cent. of the rainfall. As yet, however, few records are 
available as to the effect of the many variables involved. An investi- 
gation devoted to the question of evaporation from water surfaces, and 
_ from surfaces covered with bare soil and with various crops, under 
different conditions of wind, exposure, and mean temperature, would 
appear to be urgently needed. If this could be combined with an 
extension of Vermeulle’s investigation into the relationship between 
rainfall, evaporation, and run-off on watersheds of a few characteristic 
_ types, it would do much towards enabling an accurate estimate of the 
water-power possibilities of any given site to be predetermined. 
__ Even more useful results would follow the initiation of a systematic 
scheme of gauging applied to all streams affording potential power sites. 
Among other questions which are ripe for investigation may be 
mentioned :— 
1. The combined operation of steam and water power plants to 
. give maximum all-round efficiency. 
2. The relative adyantages of high voltage D.C. and A.C. 
generation and transmission for short distances. 
3. The operation of automatic and semi-automatic generating 
stations. 
Tidal Power.—The question of tidal power has received much atten- 
tion during the last few years. In this country it has been considered 
by the Water Power Resources Committee of the Board of Trade, who 
have issued a special tidal power report dealing more particularly with 
a suggested scheme on the Severn. The outline of a specific scheme 
on the same estuary was published by the Ministry of Transport 
towards the end of 1920. 
In France a special commission has been appointed by the Ministry 
of Public Works to consider the development of tidal power, and it 
has been decided to erect a 3,000 kw. experimental plant on the coast 
of Brittany. With the view of encouraging research the Government 
proposes to grant concessions, where required, for the laying down of 
additional installations. 
The tidal rise and fall around our coasts represents an enormous 
amount of energy, as may be exemplified by the fact that the power 
obtainable from the suggested Severn installation alone, for a period 
1921 ts 
