* I wish to point out that I have made no comment on the use by the lecture of the word "efficiency" in a popular manner instead of in the strict sense of ratio of output to input which is more usual in scientific meetings as distinct from other occasions.

* The total power plant weight may be divided into four terms:-

i. Bare motor weight . . . . . . . . . . . . . . = m

ii. Percentage allowance for revolution counters, petrol pipes, controls, etc., etc. . . . . . . . . . . . . = m ×a%

iii. Oil and petrol supply per hour × hours' duration . . . . = p ×t

iv. Percentage allowance for tanks and tank supports . . . . = p ×t x b%

The first two terms an roughly proportional to horse-power, but independent of duration of flight.

The second two terms are proportional to horse-power × hours.

Hence total weight Z₁= m(1+a%)+pxt(1+b%)

For geometrically similar machines the glider Weight inercases as the cube and the resistan ce of the members to stresses only as the square of the dimensions.

Hence to maintain the same factors of safety the total load must increase only as the square of the linear dimensions, that is as the surface.

Hence the loading c must remain constant.

If the members are tubes or shaped out girders the strength of the O I members and the glider weight may both be increased in the same proportion and the loa<linι c, may also he increased in the same proportion to a close approximation without appreciably altering the external dimensions of the aeroplane.

All these conditions are implied in the equations ( 2) ancl (3).

If , however, the members are solid, their resistance to compression and tension increases as the square of their thickness, but their resistance to buckling as the cube, and the above equations become modified.

page 247 note [This idea has often been suggested for aerial work, e.g., Duray's reversing propeller.-Ed.]