To come back to an analogy used in a previous chapter, you cannot measure all the qualities of an automobile with a ten-foot rod. Your ten-foot rod will tell you whether the wheel base is 120 inches or more or less than that. It will not tell you how much above or below 120 inches. If it be necessary for you to know that, you must provide yourself with a longer or more minutely graded measuring implement; but because the ten-foot rod does not at a glance disclose to you all that you wish to know about a particular automobile, you do not, therefore, either discredit the ten-foot rod as a measuring implement or declare that the automobile cannot be measured except by the unaided human eye. The limitations of the ten-foot rod are perfectly obvious to you; and so, too, are the complexities of the automobile, which require a variety of instruments and tests for their proper gauging and measurement. So before you undertake to form a judgment as to the ability of a particular automobile, you either measure it yourself or, as a matter of practice, you have it measured for you by a competent engineer. You do not necessarily inquire, if you have confidence in the engineer, as to precisely what dimensions and what materials he found in every part of the car, but you respect his conclusions, knowing that they are based upon the most precise and accurate measurements possible with the aid of such instruments as science has been able to devise, and you are satisfied that the conclusions form an accurate estimate of the machine’s qualities. The engineer who sets out to measure an automobile in all of its capacities and powers must provide himself with tachometers for measuring the engine’s revolutions, dynamometers for testing its tractive force, micrometer calipers for gauging the bore and the stroke, thermometers for measuring its temperature, galvanometers for testing its magneto and battery, and hundreds of other instruments, the readings of which must be assembled and studied by means of complex, comparative mathematical formulas before he can tell you what a particular automobile will do.