30 
BULLETIN 102, UNITED STATES NATIONAL MUSEUM. 
been intimated earlier in this paper and much of the matter is so 
universally familiar as scarcely to require even summary treatment. 
For the latter reason, emphasis will be placed not so much upon the 
importance of these products as indicated by the past or even the 
present, as upon their potentialities which the course of affairs are 
bound to bring out, granted a continued supply. This, in point of 
fact, is the fairer method, for we are trying to measure off the future 
significance of petroleum products against the impending inadequacy 
of the resource. 
yea R 5 
•O 20 30 40 30 60 
Fro. 9. — Curve showing the usual decline in oil-field production after the period 
OF MAXIMUM OUTPUT IS REACHED. AFTER RALPH ARNOLD, TlIE PETROLEUM RESOURCES 
of the United States, Smithsonian Report for 1916, p. 283. Compare this theo- 
retical CURVE OF FINAL DECREASE WITH THE PRODUCTION CURVES SHOWN IN FIG. 7. 
Gasoline is responsible for the most significant mechanical devel- 
opment of the twentieth century — the internal-combustion engine . 1 
The growing use of this device for generating power, with its great 
efficiency and adaptability to small movable units, such as the auto- 
mobile, has colored the whole face of modern civilization. Because 
of it, a wholly new type of automotive transportation has grown up, 
at a time when the long-established methods of cumbersome, coal- 
energized haulage were beginning to impose a serious restriction 
upon the growth of industrialism and centralization . 2 
1 Unless, indeed, the internal-combustion engine has been responsible tor the develop- 
ment of gasoline ! As a matter of fact, this type of engine was well known before 1900, 
although its growth has been most striking during the past 18 years. 
2 The electric motor, of course, has shared with the internal-combustion engine the 
credit of partly freeing transportation from the restrictions inherent in the necessity 
for hauling the source of its energy in bulky form. Further turns of this important 
consideration are followed in Bulletin 102, part 5, of this series. 
