The boys promised they would, and added the small but compact automatic camera to their luggage as they started off.
This camera worked by compressed air. There was a small motor inside, operated by a cylinder of air that could be filled by an ordinary bicycle pump. Otherwise it was just like the other moving picture cameras.
There was the upper box, in which was wound the unexposed reel of film. From this it went over a roller, and the cog wheel, which engaged in the perforations, thence down by means of the "gate," behind the lens and shutter. There two claws reached up and grasped the film as the motor operated, pulling down three-quarters of an inch each time, to be exposed as the shutter was automatically opened in front of the lens.
Each one of the thousands of moving pictures, as I have explained in previous books, is three-quarters of an inch deep, though, of course, on the screen it is enormously enlarged.
After the film has been exposed, three-quarters of an inch at a time, it goes below into another light-tight box of the camera, whence it is removed to be developed and printed. The movement of the film, the operation of the claws and the opening and closing of the shutter, making it
