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222 absolutely helpless, but because it is one of the fruitful causes of rapid tyre wear.

When a car weighing with its passengers one ton is stopped from a speed of 20 miles an hour in a length of say 15 yards, the accumulated energy in the mass in motion is about 13 tons, and this is dissipated by work done on the tyres. It is remarkable that even the best of tyres stand this enormous strain as they do. At 20 miles per hour the car travels 45 feet in the time taken to stop it, but the Automobile Club trials of January 1902 show that the distance in an assumed emergency stop may be much shorter than this, and it will not be an exaggeration to assume that the car may be stopped in 35 feet or in from 2$1⁄2$ to 3 seconds; and in this space the wheels will have made only from four to five revolutions, according to their diameter.

The whole, then, of the work, equal to that of raising a ton 13 feet high, is done by the tyre surfaces in four or five turns, or less than three seconds. This statement is sufficient to enable even those who have the very least acquaintance with mechanical matters to appreciate the danger and the costliness of the injudicious driving that leads to the abuse of the brakes. It may be desirable to record here that the Automobile Club brake trials above referred to showed that on a flat and nearly dry good road a car could be stopped at the speeds and in the car lengths given below:—

From 11 to 14 miles per hour in 1$4⁄5$ car length.

From 15 to 17 miles per hour in 2 car lengths.

From 18 to 20 miles per hour in 2$3⁄4$ car lengths.

From 20 to 24 miles per hour in 3$1⁄2$ car lengths.

Wheels.—For voiturettes there does not appear to be any structural superiority in wood wheels, making them in this respect preferable to well-made and well-proportioned wheels of the cycle type. They are a little more easily cleaned, and are, perhaps, neater in appearance; but even this is doubtful in very light cars. For the heavier cars the wood wheels of