The above is a brief and rough statement of some principles of aviation which have been ascertained by long experiment and the labour of many minds. It is by experiment that flight has been achieved. The Newton who shall reduce all the observed phenomena to a few broad and simple laws is yet to come. A bird is simpler than an aeroplane in that its wings both support it and drive it forward, whereas all aerial machines, both those that are heavier than air and those that are lighter than air, are at present driven forward by the thrust of an airscrew, revolving at the rate of some twenty to thirty times a second. There are only two kinds of flying machine, the lighter than air and the heavier than air, of which two kinds the simplest types are the soap-bubble and the arrow. These two kinds have often been in competition with each other; and their rivalry, which has sometimes delayed progress, still continues. The chief practical objection to machines lighter than air is that they are buoyed up by vulnerable receptacles containing hydrogen or some other highly inflammable gas. As soon as helium, which is a light non-inflammable gas, shall be produced in quantity at a reasonable expense, this objection will be lessened. The advantage of the lighter-than-air, or floating, machine over the heavier-than-air, or soaring, machine is that it can remain stationary in the air without loss of height, and that its great size and lifting power enable it to supply comfortable quarters for its staff, who not only travel in it, but, if need be, can inhabit it for days. The airship has a promising future, but it can never wholly supersede the soaring machine, which is heavier than air, and flies as birds fly.