260 GEOLOGICAL SURVEY OF THE TERRITORIES. 



The rough estimate I made of the volume of water in the Korth 

 Platte, near the Old Bridge, above Fort Fetterman, showed a discharge 

 of about fourteen hundred cubic feet per second. The width at a nar- 

 row and somewhat rapid point was supposed to be about one hundred 

 and sixty feet ; the average depth two feet ; the rate of the current 

 about three miles to the hour. The river at this season (August) was 

 low — at most, not more than two-thirds or one half its usual size in the 

 spring. Supposing it to have been two-thirds, this would give, as the 

 average discharge during the irrigating season, two thousand one hun- 

 dred cubic feet per second. Calculating by the rule given by Captain 

 Smith as applicable to the dryer sub-Himalayan districts — that one 

 cubic foot per second irrigates two hundred and eighteen acres — we 

 obtain the following result : As but ten-sixteenths of the water required 

 there is necessary here, it follows that one cubic foot will irrigate 348.8 

 acres, or, in round numbers, three hundred and fifty acres. This gives 

 the total amount which can be watered during the season of one hun- 

 dred and fifty days, by a canal drawing off the water at this point, at 

 seven hundred and thirty-five thousand acres, or nearly eleven hundred 

 and fifty square miles. As no area, when most densely populated and 

 in the highest state of cultivation, will ever require more than one-half 

 of the land to be absolutely and fully watered each year, we may esti- 

 mate the territory that can be brought into use by such a canal at one 

 million four hundred and seventy thousand acres, or two thousand three 

 hundred square miles; which would add at least $25,000,000 to the 

 value of our real estate, and fully $10,000,000 to the yearly value of our 

 productions. 



Calculating by the method which Marsh has adopted, the result is 

 very nearly the same. The daily distribution would be .067 of an inch ; 

 at which rate the area irrigated would be eleven hundred and seventy 

 square miles, or about seven hundred and forty-nine thousand acres. 

 These results show a difference of less than two per cent., which is 

 somewhat remarkable considering the entirely different stand-points 

 from which they start. 



It may be argued that this calculation makes no allowance for the 

 loss ; but that it is based upon the assumption that all the water is made 

 effective. This is a mistake, for it is based upon actual experiments, 

 showing the flow of water necessary to irrigate a given area, including 

 all loss except the absorption by the bottom and walls of the canals ; 

 and this in India, even in the sandy districts, does not exceed five per 

 cent. 



This calculation is certainly a safe one, as the rain-fall is reduced 

 nearly one-third below that given by the authorities on the subject, and 

 the lowest figures are used in regard to every other item. The point of 

 the Platte selected for observation was favorable, as it presented about 

 an average of velocity. Taking the average fall at only seven feet to 

 the mile, if we follow the rule given by Dwyer in his "Treatise on 

 Hydraulic Engineering," the velocity would be about two hundred and 

 ninety-five feet per minute, while I have assumed it to be but two hun- 

 dred and sixty four feet. 



Another point connected with this subject, but upon which I have no 

 information, is certainly worthy of investigation ; it is this : What will 

 be the result in the bed of the stream below the point from which the 

 water is drawn % Will it contain only the water furnished by the trib- 

 utaries entering into it below this point ? In the tract of country in 

 India lying along the base of the Si walic Hills, both east and west of the 

 Ganges, it has been noticed that, although all the water was drawn 



