RETURNS which have recently been issued by the Geological Survey of the United States Department of the Interior (Report No. 50,669) afford some interesting particulars of recent developments in the utilisation of the water power resources of the country. Up to Jan. 1, 1931, the total capacity of water-wheels installed in water power plants of 100 horse power or more was nearly fifteen million (14,884,667) horse power, representing an increase of more than a million (1,076,889) horse power, or 7.2 per cent, during the year 1930. In an article in NATURE for April 18, the corresponding figures for Canada were shown to be 6,125,000 horse power and 397,850 horse power. An estimate based on present practice in the installation of plant for the utilisation of water power indicates that about nineteen per cent of the available resources in the United States have been exploited, as compared with about fourteen per cent in Canada.