“Yes,” says Captain Hodgson, answering my thought, “Castelli thought he'd discovered the secret of controlling aeroplanes when he'd only found out how to steer dirigible balloons. Magniac invented his rudder to help war-boats ram each other; and war went out of fashion and Magniac he went out of his mind because he said he couldn't serve his country any more. I wonder if any of us ever know what we're really doing.” “If you want to see the coach locked you'd better go aboard. It's due now,” says Mr. Geary. I enter through the door amidships. There is nothing here for display. The inner skin of the gas-tanks comes down to within a foot or two of my head and turns over just short of the turn of the bilges. Liners and yachts disguise their tanks with decoration, but the G.P.O. serves them raw under a lick of grey official paint. The inner skin shuts off fifty feet of the bow and as much of the stern, but the bow-bulkhead is recessed for the lift-shunting apparatus as the stern is pierced for the shaft-tunnels. The engine-room lies almost amidships. Forward of it, extending to the turn of the bow tanks, is an aperture--a bottomless hatch at present--into which our coach will be locked. One looks down over the coamings three hundred feet to the despatching-caisson whence voices boom upward. The light below is obscured to a sound of thunder, as our coach rises on its guides. It enlarges rapidly from a postage-stamp to a playing-card; to a punt and last a pontoon. The two clerks, its crew, do not even look up as it comes into place. The Quebec letters fly under their fingers and leap into the docketed racks, while both captains and Mr. Geary satisfy them selves that the coach is locked home. A clerk passes the way-bill over the hatch coaming. Captain Purnall thumb-marks and passes it to Mr. Geary. Receipt has been given and taken. “Pleasant run,” says Mr. Geary, and disappears through the door which a foot high pneumatic compressor locks after him. “A-ah!” sighs the compressor released. Our holding-down clips part with a tang. We are clear. Captain Hodgson opens the great colloid underbody porthole through which I watch over-lighted London slide eastward as the gale gets hold of us. The first of the low winter clouds cuts off the well-known view and darkens Middlesex. On the south edge of it I can see a postal packet's light ploughing through the white fleece. For an instant she gleams like a star ere she drops toward the Highgate Receiving Towers. “The Bombay Mail,” says Captain Hodgson, and looks at his watch. “She's forty minutes late.” “What's our level?” I ask. “Four thousand. Aren't you coming up on the bridge?”