watched soaring birds, and had seen how they could swim up and down in the air without any sensible motion of the wings. When the right proportions of the machine are found out, and men by long practice have attained to skill and experience, we may perhaps, he thinks, be able to imitate the birds. If, after all, it be found that some greater motive power is required, we must not despair of the invention of such a power. The main difficulty will be not so much in maintaining the machine in flight as in raising it from the ground. 'When once it is aloft in the air, the motion of it will be easy, as it is in the flight of all kind of birds, which being at any great distance from the earth, are able to continue their motion for a long time and way, with little labour and weariness.' The right proportion of the wings, both for length and breadth; the special contrivances necessary for ascent, descent, or a turning motion--these and many more such questions can only be resolved, he maintains, by particular experiments. The sails of ships have been perfected by degrees, and the attempt to fly must meet with many difficulties and inconveniences for which only long experience and frequent trial can suggest a remedy. So far Wilkins went; and he went no farther. His speculations, however, made a deep impression on his own age, gave a bias to the researches of his fellows, and, incidentally, aroused a storm of ridicule. When Joseph Glanvill, in his vigorous little treatise called _Scepsis Scientifica_ (1665), wrote a forecast of the possible achievements of the Royal Society, he borrowed his hopes from Wilkins. 'Should these heroes go on', he says, 'as they have happily begun, they will fill the world with wonders, and posterity will find many things that are now but rumours, verified into practical realities. It may be, some ages hence, a voyage to the southern unknown tracts, yea, possibly the Moon, will not be more strange than one to America. To them that come after us it may be as ordinary to buy a pair of wings to fly into remotest regions, as now a pair of boots to ride a journey. And to confer at the distance of the Indies, by sympathetic conveyances, may be as usual to future times, as to us in a literary correspondence. The restoration of grey hairs to juvenility, and renewing the exhausted marrow, may at length be effected without a miracle; and the turning the now comparative desert world into a paradise, may not improbably be expected from late agriculture.' Again, when Sir William Temple, some thirty years later, cast contempt upon the Moderns in his _Essay of Ancient and Modern Learning_, it was the speculations of Wilkins that provoked his keenest satire. 'I have indeed heard of wondrous Pretensions and Visions of Men, possess'd with Notions of the strange Advancement of Learning and Sciences, on foot in this Age, and the Progress they are like to make in the next; as, the Universal Medicine, which will certainly cure all that