The pressure of the air in the tunnel depends upon the depth and as a rule it varies between 20 and 40 or even more pounds per square inch above atmospheric pressure. Working in compressed air causes a peculiar disease commonly known as “bends” or “caisson disease” often proving fatal. To prevent and remedy the disease, the engineers should order a set of rules to be strictly observed. The preventative measures should be, first, to employ only sober, strong and healthy men, never one who has not successfully passed the examination of the attending physician; second, to order the lock tenders never to allow any man in or out of the tunnel unless he has spent at least ten minutes within the locks. Both compression and decompression should be thorough and it cannot be in less than this time. A stop of only a few minutes in the locks is not sufficient and this incomplete compression or decompression is the real cause of the bends. The men become careless after they have been in the compressed air for some time, and they try to reduce this tiresome operation to a minimum, hence the duty of the engineer to strictly enforce this rule. The remedial measures should consist of constant medical attendance near the shafts and the erection of a compressed air hospital where the men affected by bends for lack of decompression may be attended and cured. THE HUDSON RIVER TUNNELS OF THE PENNSYLVANIA RAILROAD.[13] The tunnels constructed under the Hudson River for the Pennsylvania Railroad, consist of two parallel tubes driven side by side 14 ft. apart. The tubes are of circular cross-section, 23 ft. exterior diameter, and are lined with cast-iron rings. The tunnels were driven from two shafts, one on the eastern shore of the Hudson River near 32nd St. and 11th Ave., New York; the other at Weehawken, New Jersey, near the piers of the Erie Railroad. The horizontal distance between the shafts was 6550 ft. The permanent one at Weehawken was built on a square plan, 130 ft. to a side. It was lined with concrete masonry and the walls were battered in such a way as to become the shape of an inverted frustum of a pyramid. It was provided with five openings at the bottom, four of these are used by trains that run in the open, the fifth one leads to a power house near by. During the construction of the tunnels one-third of this shaft was used for the land portion of the tunnel under Bergen Hill, while the remaining two-thirds were devoted to the construction of the tunnel under the river. The working shaft on Manhattan Island was a side shaft of rectangular plan 30 ft. by 22 ft., the tunnel proper being connected by two drifts 10 ft. by 10 ft. each. The shield rooms 23 ft. long, were situated on both sides of the river just in front of the shafts. On the New York side, the shields, one for each tube, were built inside the iron lining of the shield chamber, and the hoisting tackle was slung from the iron lining. The erection on the