exceeded. Flying is comparable to hill climbing on a road automobile where represents the gradient, this being about 1 in 5 or 1 in 6, and where the transmission efficiency is limited to about 66 per cent. The velocity limit rests entirely on the weight per horse-power, the aerodrome being presumed designed for least resistance; any continued improvement in prime movers, tending to a reduction of weight, will react in the direction of rendering higher aerial speeds practicable.
If it should be found possible to “soar” on a large scale after the manner of an albatros or gull, the limitation of range may, in certain exceptional cases, be partially or wholly removed.
It may be noted that on the liquid hydrogen estimate of maximum possible range, no allowance has been made for the possible power to be derived directly from the expansion prior to combustion. We have also omitted to discuss the possible increase of thermal efficiency theoretically available by the employment of the low temperature of the boiling point of hydrogen as a refrigerator, that is, as the temperature at which the heat engine discards. The use of liquid hydrogen is at present too daring and distant a suggestion to be taken quite seriously; it has here been put forward merely as representing the maximum known fuel value in a possibly available form.
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