“Are you going to take a camera with you, boys?” asked Mr. Ringold, as Joe and Blake were saying good-bye to their friend, preparatory to making a brief stay in San Francisco.  
“A camera? No. Why?” inquired Blake.
 
“Well, I happen to need some San Francisco street scenes for one of the dramas,” went on the man; “and it occurred to me that you could get them when you weren’t busy.”
 
“Of course we could,” answered Joe. “We can take the automatic, and set it up wherever you say, and go looking for that agent. When we come back we’ll have all the pictures we need.”
 
“Good!” exclaimed Mr. Ringold. “Try that, if you don’t mind. Get some scenes down in the financial district, and others in the section. Then, as long as you have to go to the shipping offices, get some there.”
 
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 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 . 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 89possible to take sixteen pictures a second, was, in this camera, all controlled by the air motor.
 
Joe and Blake found much to amuse them in San Francisco, which they had never before visited. They were a bit “green,” but after their experiences in New York they had no trouble in finding their way around.
 
“We’d better go to some hotel, or boarding house,” suggested Joe, after their arrival. “Pick out one where we can leave the camera working while we’re gone.”
 
They did this, being fortunate enough to secure rooms in a good, though not expensive, hotel near the financial district. One of their windows looked directly out on a busy scene.
 
“That’ll be just the place, and the sort of scene Mr. Ringold wants,” declared Blake. “Let’s set the camera there on the sill and see what it gets. The light is good to-day.”
 
It was, the sun shining brightly, and being directly back of the camera, which would insure the proper illum............