The outline of the argument, the reader will remember, was as follows:
1. There is a limit to the size and weight of any locomotive machine, whether natural or artificial. This limit is the result of the law that while the strength of material and force of all kinds, whether by muscular contraction or by steam pressure, increase as the square of the diameter of muscle or of piston, the weight of the machine varies as the cube of the diameter of all parts. Thus with increasing size, weight must quickly overtake and pass beyond strength. This limit varies with the kind of machine. The limit of an efficient walking machine was probably reached in the largest land animals of previous geological times. For a rolling machine like a locomotive engine or a bicycle, where the weight is supported on wheels, or in a swimming machine where the weight is supported by water, and where, therefore, in both cases the whole energy is expended in progression, the limit is much higher; and, therefore, a locomotive engine and a whale may be heavier than any walking animal.
2. The limit of weight of a flying machine is very much lower than that of either a swimming, rolling, or a walking machine. The limit of an efficient, manageable flying machine is in fact reached in the largest birds, such as in the condor among long-winged and the bustard among short-winged birds, and to our surprise we find it only about fifty pounds. The condor can barely lift himself from the ground, although when well up he sails with ease and grace. There are, indeed, still larger birds, like the ostrich, but they can not fly. True, their wings are rudimentary, but they have become so only because these birds have passed beyond the flying limit.
3. Now, a bird is admirably constructed for economy of force. Not only is everything sacrificed to the one supreme object of flying, but the animal machine, using fats and starch for fuel and getting energy through the mechanism of nerve and muscle, is admittedly more economical and efficient—i. e., will develop more force and do more work for the same weight of fuel and machine—than any artificial machine yet devised. It seems hopeless to surpass it. Therefore, the weight of a machine that will be able to lift itself in the air can not exceed fifty or a hundred pounds.
4. But it is idle to talk of a flying machine with fuel and engineer and freight being less than many times this limit. Therefore, a flying machine which is anything more than a toy is impossible.
Such is a bare outline of the argument which seemed then—and to a large extent seems now—irrefutable. But Langley's recent experiments certainly put the question in a new and somewhat more hopeful light; and renewed reflection on the whole