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 for commercial purposes and has become increasingly so with the enormous extension of the use of motor cars.

However, we are now on the eve of a new epoch in this line through the invention of Dr. Rudolph Diesel, the German engineer, who so mysteriously disappeared last October on his voyage to England.

It is now 20 years since Dr. Diesel published the first sketch of his remarkable theory and of the motor which was to realize his idea. The motor is simplicity itself. Every schoolboy knows that if air is compressed very sharply it becomes hot and can be used to explode powder, etc., in a tube. Dr. Diesel's plan was to use the stroke of the piston to compress a considerable volume of air into a very small space, so as to put it under a very high pressure; and at the instant the pressure reached a maximum, to force into this chamber a jet of vaporized oil. The compression was to be so high that the air would instantly ignite the oil and burn it under highly favorable conditions. It is a true burning, and not an explosion, as in the ordinary gasoline motor of the automobiles. His idea was taken up by some of the engine works in Germany, but it required fully four years to effect a commercial device. The superiority of the new motor was evident from the first. Actually it realized a full third of the theoretical heat energy of the oil, and this latter did not need to be gasoline or other expensive essence, but could be ordinary crude oil, such as comes out of the earth. The device is self-igniting, requires no auxiliary system and little or no attention.

It was soon found, however, that the new motor had to be made with exceptional care, and that, therefore, the cost of its development for commercial use was high. The fact that capitalists are not interested in progress as such, but in profit, explains why it is that, in spite of the great economies it achieves, the Diesel motor is now only becoming widely known.

In Germany, at the current price of crude oil, the