The heavy iron-tipped arrow rests in the shallow wooden trough or groove which travels along the stock.
The trough has a strip of wood, in the form of a keel, fixed beneath it. This keel travels to or fro in a dove-tailed slot cut along the upper surface of the stock for the greater part of its length. (F, [fig. 14], p. 23.)
The arrow is laid in the trough before the bow-string is stretched. (A, B, [fig. 14], p. 23.)
The balista is made ready for use by turning the windlass. The windlass pulls back the sliding trough, and the arrow resting in it, along the stock of the engine, till the bow-string is at its proper tension for discharging the projectile. ([Fig. 13], p. 21.)
As the trough and the arrow are drawn back together, the arrow can be safely laid in position before the engine is prepared for action.
The catch for holding the bow-string, and the trigger for releasing it, are fixed to the solid after-end of the wooden trough. ([Fig. 14], p. 23.)
The two ratchets at the sides of the after-end of the trough travel over and engage, as they pass along, the metal cogs fixed on either side of the stock. ([Fig. 14], p. 23.)[10]
[10] When the bow-string has been released and the arrow discharged, the ratchets are lifted clear of the cogs on the stock of the engine. This allows the trough to be slid forward to its first position as shown in A, B, [Fig. 14]. It is then ready to be drawn back again for the next shot.
By this arrangement the trough can be securely retained, in transit, at any point between the one it started from and the one it attains when drawn back to its full extent by the windlass.
As the lock and trigger of the balista are fixed to the after-end of the sliding trough ([fig. 14], p. 23), it will be realised that the arrow could be discharged at any moment required in warfare, whether the bow-string was fully or only partially stretched.