The Crookes radiometer is a glass bulb containing a little ‘mill’ that spins in sunlight. Each vane of the mill is blackened on one face, which warms up compared with the other, reflective, face. Gas molecules hitting a vane edge flow back along it from the cold side to the hot one, and drive the vane by their reaction. This ‘thermal transpiration’ is most efficient when the mean free path of the gas molecules matches the vane thickness, which for vanes 0.1 mm thick implies a low pressure around 5 pascals.

Daedalus once devised a radiometer helicopter rotor, to fly in the low pressures at high altitude. He now points out that the mean free path in air at ground level is about 10 nm; so a radiometer could spin in ambient air if it had blades 10 nm thick. No macroscopic blade could be made so thin. But imagine, says Daedalus, a 10-nm wire, heated by a current or radiative absorption, and moving sideways through ordinary air. Its leading edge would be selectively cooled by the impacting air molecules, thus establishing a front-to-rear temperature difference. So those molecules would be impelled backwards by thermal transpiration, driving the wire forward and augmenting its speed. Given a starting push, the wire would continue to accelerate in that direction, like a ramjet.

So Daedalus's ‘ram radiometer’ is simply a fine grid of 10-nm wires in a duct about a millimetre across. It is the smallest possible aero-engine. Several such engines distributed about a tiny winged airframe will complete an ‘artificial fly’. Modern microfabrication methods will shape the wire grids, form the tiny photovoltaic cells that power them, and lay down the steering electronics. This will turn different engines on and off in response to infrared digital codes beamed at the photocells.

Powered by daylight and controlled by an infrared unit rather like a TV-channel changer, the artificial fly will be an elegant and unobtrusive remote-sensing and transmitting gadget. Carrying a tiny microphone or a set of chemical sensors, it could snoop and soar, sniffing out leaks in a chemical plant, conversations in a crowd, or the microclimate in a forest. Daedalus likes the idea of forming a number of them into a personal defence squadron against real flies and mosquitoes. They could defeat the pests by ramming — insect wings are notoriously vulnerable to mechanical damage — or even, with sufficiently cunning control programs, by seduction.