From this time on until 1893, experiments continued to be made with rubber-driven models, of which, as has been stated, nearly 40 were constructed, some with two propellers, some with one; some with one propeller in front and one behind; some with plane, some with curved, wings; some with single, some with superposed, wings; some with two pairs of wings, one preceding and one following; some with the Pénaud tail; and some with other forms. A few of these early forms are indicated on the accompanying Plates 1 to 4, but it does not seem necessary to go into the details of their construction. No. 11 with which an early flight was made, closely resembles the Pénaud model. No. 13 has two propellers, one in front and one behind, with a single wing. No. 14 has two propellers, nearly side by side, but one slightly in advance, with a single wing and a flat horizontal tail. No. 15 has one leading propeller and two broad wings, placed one behind the other. No. 30 has the propeller shafts at an angle, and one pair of wings. No. 31 has the propeller shafts at an angle, and two pairs of wings superposed. The wings in general were flat, but in some cases curved. The rubber was usually wound to about 100 turns, and trouble continually arose from its “kinking” and unequal unwinding, which often caused most erratic flights. It is sufficient to say of these that, rude as they were, much was learned from them about the condition of the machines in free air, which could never be learned from the whirling-table or other constrained flight. The advantages and also the dangers of curved wings as compared with plane ones, were shown, and the general disposition which would secure an even balance, was ascertained; but all this was done with extreme difficulty, since the brief flights were full of anomalies, arising from the imperfect conditions of observation. For instance, the motor power was apparently exhausted more rapidly when the propellers were allowed to turn with the model at rest, than when it was in motion, though in theory, in the latter case more power would seem to be expended and a greater speed of revolution obtained in a given time. The longest flights obtainable did not exceed 6 or 8 seconds in time, nor 80 to 100 feet in distance, and were not only so brief, but, owing to the spasmodic action of the rubber and other causes, so irregular, that it was extremely difficult to obtain even the imperfect results which were actually deduced from them. [p014] ABBREVIATIONS AND SYMBOLS EMPLOYED