A header is the act of “going down on the other side,” spoken of in a former chapter, or, more definitely, it is the projection of the rider over the handle-bar to the ground in advance of his machine. It is a simple process, being a mere application of the physical forces of gravity and momentum. A moving body tends to keep moving in a line until stopped or deviated by some counteracting force. In riding a cycle a certain momentum is acquired and kept up against the resisting forces of friction, impact of air, road resistance, etc. Headers are a result of a counteracting force, generally caused by sudden impact against a stationary obstacle on the road, or by the forward wheel becoming suddenly locked through a failure in the axle-bearings to work, or by some clog in the wheel preventing it from revolving freely through the fork in which it is hung. There are modifications of the header action even in machines of the same sizes of wheels and same rake,—rake being a term recognized to express the angle of the front fork from the vertical. This rake has to do with the liability to headers only in so far as it regulates the centre of gravity of the system, “more rake” generally meaning that the rider is farther behind the vertical line through the front wheel axle. In order to take a header, a certain centre of gravity must get beyond a certain line. This centre of gravity will vary in position in different machines, and the modifications spoken of cause the line to move in a way which is, I think, sometimes overlooked. If we discuss the crank Ordinary, it will be noticed that when the front wheel is stopped in its forward progress, the frame of the machine together with the rider and all other parts of the system revolve about the centre of the wheel and cause an action within the system, the same as that of the forward wheel revolving backward through the fork. Now, it is just when such backward motion is prevented, that the gravity line moves and alters the conditions, decreasing the liability to headers. If the forward wheel can revolve backward through the fork, then, in taking a header, the system, exclusive of the forward wheel, will revolve about a point in the wheel centre; but if it cannot so revolve, then the entire system, including the forward wheel, must all tend to revolve about the point of contact of the wheel with the ground. Now, it will be seen in the latter case, or anti-header machine as we shall call it, that as the system tends to revolve about the point of contact, such point will constantly change; in other words, the wheel must roll onward, and the point of contact will therefore advance. [Illustration: Header action, smooth road.]