We are going to see something of the construction of one of these great railroad bridges. Let us begin at the beginning, and see the men, as they work upon the foundations of abutments and of piers--many times hundreds of feet under the waters of the very stream that they will eventually conquer. For months this important work of getting a good foothold for the monster will go forth almost unseen by the workaday world--by the aid of the great timber footings, which the engineer calls his caissons. These caissons (they are really nothing more or less than great wooden boxes), are slowly sunk into the sand or soft rock under the tremendous weight of the many courses of masonry. They sink to solid rock--or something that closely approximates solid rock. We are going down into one of the caissons that form the foothold of a single great pier of a modern railroad bridge; we are going to stand for a very few minutes under air-pressure with the "sand-hogs"--men whom we first came to know when we studied the boring of a tunnel. Air pressure spells danger. It takes a good nerve to work high up on the exposed steel frame of some growing bridge, but the bridge-builders have air and sunlight in which to pursue their hazardous work. The sand-hog has neither. He toils in a box down in the depths of the unknown, working with pick and shovel under artificial light and under a pressure that becomes all but intolerable. The knowledge that the most precious and vital of all man's needs--fresh air--is controlled by another, and through delicate and intricate mechanism, cannot add to his peace of mind. No wonder, then, that it is the highest paid of all merely manual work. The sand-hog working 50 feet below datum is paid $3.50 for an eight-hour day. But 50 feet is but the beginning to these human worms, who burrow deep into the earth. Below it they first begin to divide their day into two working periods. The air begins to count, and men with steel muscled arms must rest. As they approach 80 feet below datum--the engineers' phrase for sea level,--they are working two periods each day of one hour and a half apiece, while their daily pay has risen to $4. There is your rough arithmetical law of sand-hogs. As your caisson goes down so does the length of your working-day decrease; inversely, their air pressures and the pay of the men increase. The cost? The cost leaps forward in geometrical progression. It is the owner's turn to groan this time. One hundred feet is the limit. At 100 feet the air pressure is more than 50 pounds to the square inch--three additional atmospheres--and the limit of human endurance is reached. The men work two shifts of forty minutes each as a daily portion and the law steps in to say that they must rest four hours between the shifts. They are paid $4.50 for that day's work--which means something more than $4 an hour for the time that they are actually at work in the caisson.