“A hundred and twenty miles an hour,” I hear Brennan saying, in that sad voice of his; “or maybe two hundred.  That’s a detail.”

In the back of the cab were broad unglazed windows, through which one could watch the tangle of machinery.  Dynamos are bolted to the floor, purring under their shields like comfortable cats; abaft of them a twenty-horse-power Wolseley petrol-engine supplies motive power for everything.  And above the dynamos, cased in studded leather, swinging a little in their ordered precession, are the two gyroscopes, the soul of the machine.  To them she owes her equilibrium.

Of all machines in the world, the gyroscope is the simplest, for, in its essential form, it is no more than a wheel revolving.  But a wheel revolving is the vehicle of many physical principles, and the sum of them is that which is known as gyroscopic action.  It is seen in the ordinary spinning top, which stands erect in its capacity of a gyroscope revolving horizontally.  The apparatus that holds Brennan’s car upright, and promises to revolutionize transportation, is a top adapted to a new purpose.  It is a gyroscope revolving in a perpendicular plane, a steel wheel weighing three quarters of a ton and spinning at the rate of three thousand revolutions to the minute.

Now, the effect of gyroscopic action is to resist any impulse that tends to move the revolving wheel out of the plane in which it revolves.  This resistance can be felt in a top; it can be felt much more strongly in the beautiful little gyroscopes of brass and steel that are sold for the scientific demonstration of the laws governing revolving bodies.  Such a one, only a few inches in size, will develop a surprising resistance.  This resistance increases with the weight of the wheel and the speed at which it moves, till, with Brennan’s gyroscopes of three quarters of a ton each, whirling in a vacuum at three thousand revolutions per minute, it would need a weight that would crush the car into the ground to throw them from their upright plane.

Readers of MCCLURE’S MAGAZINE were made familiar with the working of Brennan’s gyroscope by Mr. Cleveland Moffett’s article in the issue of December, 1907.  The occasion of that article was the exhibition of Brennan’s model mono-rail car before the Royal Society and in the grounds of his residence at Gillingham.  For a clear understanding of the first full-sized car, it may be well to recapitulate a few of the characteristics of the gyroscope.

When Brennan made his early models, he found that, while the little cars would remain upright and run along a straight rail, they left the track at the first curve.  The gyroscope governed their direction as well as their equilibrium.  It was the first check in the evolution of the perfect machine.  It was over ten years before he found the answer to the problem—­ten years of making experimental machines and scrapping them, of filing useless patents, of doubt and persistence.  But the answer was found—­in the spinning top.