The jumper starts at a and runs off the edge of the platform b into the air; landing on the slope below at c, he runs down it and out on the level, where he swings to a standstill at d.
The gradient of the hill above the platform is preferably not more than 20° or so, for the jumper must above all things be perfectly steady as he leaves the platform, and if the upper part of the hill is very steep the sudden change of gradient as he runs on to the platform is likely to upset his balance. The impetus can therefore be obtained more safely from a long run at a moderate gradient than from a short steep one.
Fig. 44.
The steeper the slope below the platform, other things being equal, the longer obviously will the jump be, and also the less will be the shock to the jumper as he lands, on account of the narrower angle between the ground and his course through the air. For this reason a steep gradient below the platform is an advantage, and on big jumping-hills the angle of this part of the slope is from 25° to 35°.
The dotted line in the diagram shows how, if the platform were placed just at the point where the slope becomes steeper, the length of the jump would be greater than if, as is usually the case, the platform were built farther back; at the same time, however, the shock to the jumper on landing would be increased also, for his course through the air the moment before alighting would then be more directly downwards, and when in the air he would, at his highest point, be farther from the ground.
This is one reason why the platform is usually placed farther back. There is sometimes another reason. The gradient must remain the same for a sufficient distance below the point where the jumper lands to enable him to get a steady balance on landing, and unless the steep part of the slope is very long, it may only be possible by putting the platform farther back to ensure that the jumper shall not land too near the bottom of the hill.
For the same reason (of making things easier for the jumper) the change of angle between the top part of the hill and the platform, and also between the lower part and the level outrun, should take place as gradually as possible.