684 ECONOMICAL ASPECTS 



lute minimum, the minimum of the average year, the average 

 minimum, or some other value of low discharge of the stream. 

 The selection of the particular value to be used depends upon the 

 degree of insurance of the continuity of the supply that is justified 

 by the .conditions. The added cost of the insurance of the supply 

 should be equated to the losses, direct and indirect, sustained by 

 failure of the supply. If it is planned to secure absolute con- 

 tinuity, in so far as stream-flow is concerned, it will be necessary 

 to use the absolute minimum of the stream and to use it in con- 

 nection with the maximum load that can occur upon any day 

 when the stream-flow may be lowest. This degree of insurance is 

 seldom necessary; usually it will be sufficient to use the stream- 

 flow which can be depended on for, say four years out of five; 

 in other words, to eliminate the extraordinary low discharge which 

 will occur once in every five to ten years. But on this point, as 

 in all others in connection with the matter, the decision depends 

 upon the experience and judgment of the engineer, and no hard- 

 and-fast rule can be laid down. One kind of load demands the 

 highest degree of insurance, whereas loads of a different character 

 may be satisfactorily served with a less degree of insurance. 



Secondary power is that amount which is being developed 

 above the primary, and which is only available for a certain time 

 of the year, such as eight or ten months. The continuity of this 

 power is, as a rule, not guaranteed, and the right is reserved to 

 cut off such supply upon reasonable notice. The rates for sec- 

 ondary power are, therefore, much lower than for primary power. 

 i The question of the sale of secondary power has yet not 

 reached the proportions to which it is entitled, but there is every 

 reason to believe that by careful planning of certain industries 

 quite a large amount of secondary power could be very econom- 

 ically utilized. 



The question as to what extent a power site should be devel- 

 oped depends necessarily upon the market conditions for the two 

 classes of service. It needs no argument to prove that where 

 power can be sold at a high price and conditions are favorable, the 

 development can be carried to a higher point of stream-flow than 

 where the opposite conditions prevail. Over-development, how- 

 ever, may entail fixed charges which will make the earning of a 

 surplus only a speculative possibility of the distant future. On 

 the other hand, the present demand and its probable future 



