Screw-jack.
A lifting-jack, in which the power consists of a screw rotating in a nut in the body of the tool. See jack-screw. a, screw-jack, with wooden case. b, screw-jack, with malleable iron case. c, windlass screw-jack, single purchase. d, windlass screw-jack, double purchase. In these, additional power is obtained by multiplying gears interposed between the lever and the nut which turns the screw. e, tripod-jack. f, bottle-jack. So termed from the form of the case. They are operated by a removable lever inserted in the head. It is by no means an uncommon thing for a house to be lifted bodily from its site and removed by machinery to a new one, without disturbing the inmates, but in Chicago the operation was extended to almost the whole city, in 1867-68. The early buildings were generally erected without any regard to the laws of drainage, and eventually, as the city increased in size, the evil became intolerable. By means of screws acting under balks of timber, one of the largest hotels, known as the Briggs House, was raised in 27 days to a hight of 4 feet 2 inches above its previous level. The building had a frontage of 180 feet, a depth of 80 feet, weighed 22,000 tons, and was 5 stories high, presenting accommodation for 450 guests, none of whom were disturbed during the operation. Tremont House, another hotel of a similar size, was also raised without accident. The screws employed were about 2 feet long, 2 1/2 inches in diameter, with a pitch of half an inch. They worked in cast-iron sockets, and were moved by handspikes: 1,450 such screws and 600,000 cubic feet of timber were used in raising the Briggs House. A similar plan was adopted in July, 1868, at Boston, when whole streets of houses were raised in blocks of 6 houses together. |
| Screw-jacks. |
