216 W.TMCGEE OUTLINES OF HYDROLOGY 



mainland United States; the cost, both absolute and relative to other 

 elements of the dietary, varies widely. 



NAVIGATION 



The earliest stage in the collective utilization of water is that arising 

 in navigation: Long before written history began, the continents were 

 peopled through migrations along shores and streams, and most of the 

 primitive peoples were more or less adept in the use of water craft ; later, 

 exploration and settlement followed waterways, and the leading countries 

 of the world have adopted regulations for ocean and lake commerce and 

 liave entered on the control of rivers and the construction of canals for 

 inland navigation. The basis of the stage comprises the 142,000,000 

 square miles of water surface on the globe, and the 6,000 cubic miles of 

 river water flowing seaward annually from the 55,000,000 square miles of 

 land surface. Originally common to all, the oceans may be said to have 

 passed under commercial control, largely national and international, 

 though little progress has been made (save in certain harbors) toward 

 physical control. The rivers and lakes are generally used in a nominal 

 degree for commercial purposes by the nations in whose domains they lie, 

 and in a relatively small number of rivers (with a few canals) physical 

 control has been undertaken, usually through the transfer of material by 

 excavation and filling — that is, by processes for which moving waters are 

 the natural and by far the most effective and economical agency. 



Water affords the most economical means of transportation, and in a 

 general way oceanic commerce increases geometrically with the growth of 

 the world's population; in some cases inland navigation increases with 

 population, in others it declines either temporarily by reason of economic 

 conditions or more persistently through indifference or inefficiency of 

 states and nations. In no country has complete control of navigable and 

 source streams been undertaken in the interests of mankind, thoiigh in 

 America a general plan for doing so has been approved by the chief ad- 

 ministrative authorities of states and nation. 



POWER 



The next collective stage is that of power arising in natural movement : 

 Stream-flow and oceanic currents were utilized to facilitate navigation in 

 the earliest times, and many primitive peoples derived from waterfalls 

 and rapids the power required for certain rude industries; later under- 

 shot and breast and overshot water-wheels were perfected, and sluices 

 and dams and reservoirs were devised to utilize the natural head first of 



