within the cylinder and conveying the pressure produced from the heat evolved by means of a piston and linkage to the crank- shaft as a usable power than to the physical processes upon which the economical action of the motor itself depended.

No doubt, during this mechanical age some theoretical con- sideration was given to the subject, experimenters examining their designs in the light of such information as they had of the physical characteristics known at the time.

We might, as an instance of knowledge or foresight, take the inventions of Barnett, in 1838. He invented three engines, each shewing marked novelty, but the outstanding feature was the introduction of the principle of compression, which has been the mainspring of economy ever since. The mechanical age continued some time after Barnett, and various linkage devices, such as the free piston engine, were tried, until design finally settled down to the conventional crank and connecting rod of the reciprocating prime movers that we have to-day. During this period but little scientific research was carried out and this is doubtless the reason for many erroneous ideas continuing to be held of the working processes of the engine. Such an idea was that of Lenior, who supposed that the economy of the engine would be improved by a slower rate of explosion or combustion.

The advantages of compression were again stated by Schmidt in 1861, and became more widely appreciated. The pamphlet of Beau de Rochas in 1862, which has formed the basis of most internal combustion engines, stated that there should be

I. The greatest possible cylinder volume with the least cooling


2. Great rapidity of expansion.

3. The greatest possible expansion; and

4. The greatest pressure at the beginning of expansion. These, he said, meant:

I. Suction during the outstroke of the piston.

2. Compression during the following instroke.

3. Ignition and expansion during the third stroke.

4. Discharging the burnt gases on the fourth stroke.

Here we have the famous four-stroke cycle, Which was put into practical form by Otto in 1876, and is embodied in the majority of engines to-day.

The other well-known cycle, the two-stroke, was invented by Dugald Clerk in 1877. He was its most distinguished exponent, and was later to add much to our knowledge by his research work into the heat processes of the engine.