Estimates of the amounts of petroleum yielded are made by computing the amount usually produced per acre, which varies from eight hundred barrels produced in Pennsylvania, to eight thousand barrels per acre produced in Illinois. In most of the fields it is about a thousand barrels per acre. Even then the amount is extremely difficult to estimate. The Geological Survey concludes that the lowest probable calculation of the entire amount stored in the rocks of the United States is ten billion, and the highest a little less than twenty-five billion barrels. The last report officially published shows that we are producing one hundred and seventy million barrels per year. If the same rate of production continues, we might expect our petroleum to last from fifty-five to one hundred and thirty-five years, according to the amount found; but tables of statistics show that throughout the life of the petroleum industry, as much has been produced each nine years as the entire product before that time. For example, up to the present, we have produced one billion eight hundred million barrels and if the present rate continues, in the next nine years alone we shall produce an equal quantity again. The causes of such rapid growth are many. One is the great increase in the use of some of the products, such as gasolene, which has increased many fold since the automobile became popular. Another, and the greatest cause, is the ease with which any quantity of oil can be sold for cash at any time, and at prices much above the cost of production. Another reason is based upon the nature of the product. In pumping from one well oil is apt to flow in from other leases, under other farms, and exhaust them without the holders of those leases having received any compensating benefit. It is therefore necessary for each lessee to get his share before it flows away. Under these circumstances, it is impossible to prevent an entire field from being drilled over very rapidly, unless there is a combination of all the interests; or unless the law limits the amount that each producer shall extract per acre within a given time. Pennsylvania and New York have declined to one-third their former value and yet it is only seventeen years since they reached their highest point. This would seem to indicate that the life of that field will not exceed ten years. West Virginia is producing only a little more than half its former yield and is rapidly declining. Ohio and Indiana are declining more rapidly than Pennsylvania. Texas is also in the rapidly declining class, and in Kansas the production is only a fraction of what it was formerly. On the other hand, Illinois, Oklahoma, and California can be expected to increase steadily for several years.