“It must take an awful lot of power to lift an elevator,” I remarked.
“Not as much as you think. In fact, it often takes more power to ran an empty car down than a partly loaded one up.”
“Why, how can that be!’ we both exclaimed in astonishment.
“It is like this. A car is always balanced with a counterweight. The cables that run up from the top of the car pass over a set of sheaves or pulleys at the top of the shaft, and at their other ends they are attached to the counterweight. Usually the counterweight is made heavy enough to balance the weight of the car with half a load of passengers. Now, if the brakes should give way on the winding-drum at the top of the shaft while a car is standing empty half-way up the shaft, it would actually fall up instead of down, because it would be so greatly overweighted by the counterbalance. You see, all the motor has to do is to move the difference in weight between the car with its passengers and the counterweight; and this can never equal more than half the weight of the passengers. But I don’t suppose you would find our elevator system half so interesting as the one I am going to send you to. The only uncommon thing we have is an ‘air-cushion,’ but that is not very unusual any more.
“By an air-cushion, I mean,” he continued in answer to our question, “a scheme for retarding the car in case it should fall by any mischance. The bottom of each shaft is sealed in with air-tight steel doors, so as to make a rectangular pocket in which the car fits like a plunger in a cylinder. Now, if the car should drop into that pocket at high speed, it would compress the air under it Lo such an extent as to form a pneumatic cushion that would check its fall. Our highest elevator-shaft will be six hundred and eighty feet high, the highest continuous elevator-shaft in the world, and, as you can imagine, a car would be traveling if it fell that far! We don’t dare to make the stop too abrupt, for it would hurt the passengers, and then, too, it would be liable to burst out the doors, so we don’t make too close a fit of the car floor in the shaft. But that means that we have to extend the air pocket to a considerable height. On those high shafts, the air pocket extends up one hundred and thirty-seven feet, or ten stories. You could cut the cables with the car at the top of the shaft and let it fall. It would be making something like one hundred and thirty-two miles per hour when it823
