The balance between supply and demand for water is a delicate one, marked over time by political and environmental conflicts, the impacts of natural disasters and human actions, and the day-today demands for a multiplicity of uses for this most vital resource. Although a renewable resource, water is not always available to a thirsty Nation when and where it is needed, nor is it always of suitable quality for the intended use. Water must be considered as a finite resource that has limits and boundaries to its availability and suitability for use. In the United States, many existing sources of water are being stressed by withdrawals from aquifers and diversions from rivers and reservoirs to meet the needs of homes, cities, farms, and industries. Increasing requirements to leave water in the streams and rivers to meet environmental, fish and wildlife, and recreational needs further complicate the matter. After continual increases in the Nation's total withdrawal of surface water and ground water for the years reported from 1950 to 1980, water withdrawals declined and have remained fairly constant since the mid 1980's. The decline in withdrawals is especially significant in light of the fact that population continued to increase during the same period. Clearly as a Nation, we are using our surface-and groundwater resources more efficiently. This decline signals that water use does respond to economic and regulatory factors, and that the general public has an enhanced awareness about waterresources and conservation issues. As planners, managers, and elected officials wrestle with the varied watermanagement problems facing the Nation at the beginning of the new century, they need consistent information on water supply and use by State, watershed, and water-use category. This will help the Nation realize the maximum benefit from its water resources and will help strike that crucial balance between supply and demand. The U.S. Geological Survey has compiled and disseminated estimates of water use for the Nation at 5-year intervals since 1950. In 1977, the Congress expanded the Survey's water-use activities by establishing a National Water-Use Information Program, which, in cooperation with the States, collects reliable and uniform information on the sources, uses, and dispositions of water in the United States. The result of that cooperative effort is a valuable long-term data set of national water-use estimates that can be used to assess the effectiveness of alternative water-management policies, regulations, and conservation activities, and to make projections of future demands. This Circular documents water use in 1995 and identifies changes in water use that have occurred over the past 45 years.

Downloaded from iodides used in the preparation of the hydrocarbons were carefully purified by repeated fractionation, and the following are the boiling points of the specimens used.Methyl

R ecent experim ents by R oberts* have shown, not only th a t the accommoda tio n coefficients of helium and neon atom s impinging on a clean heated tungsten surface are extraordinarily low, b u t also th a t these values increase with time after cleaning th e surface. To explain this increase he suggests the gradual form ation of adsorbed films on th e surface of th e tungsten due to residual im purities in th e gas. The prim ary object in starting the investigations to be described in this paper was to gain some inform ation as to the nature of these films, th e existence of which has also been postulated by Blodgett and L angm uir.f F o r this purpose experim ents have been carried out in which th e emission of energy from electrically heated wires of platinum, a metal relatively resistant to contam ination, has been investigated under varied conditions. ApparatusA pure platinum wire about 20 cm long and 0 • 025 mm diameter was silversoldered to platinum leads of 0*4 m m diam eter and mounted loosely in a vertical glass tu b e of approxim ately 5 cm diameter. The wire was connected, b y double leads, to a Thom son bridge, by means of which the resistance could be determ ined and also controlled by suitable adjustm ent of the current. The potential drop along th e wire was measured by means of a Siemens and H alske potentiom eter. The m ean tem perature of, and the heat developed in, th e wire could th u s be determined.The tu b e containing th e wire was connected by glass tubing of 1*5 cnl bore to a flask of about 10 litres and to a pipette system which has been described by K nudsen, % by means of which small qua * ' P roc. R o y . S o c

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