There are many varieties of screw jacks on the market, all of them being adaptations of the bottle-jack before referred to. The only one we need notice here is the variation known as the windlass jack (Fig. 223). The essential difference between this and the simple bottle-jack is that the nut which the screw works in is turned by the vertical handle, by means of a pair of small toothed wheels geared in the ratio of two complete revolutions of the handle to one of the nut. In this way double the weight can be lifted with the same effort as would be used with a bottle-jack. The windlass jack is also provided with a foot lift for applying force between two objects which are too close together to admit the whole length of the jack. This consists of a projecting "foot" attached to the lower end of the screw, and arranged to work through a slot in the side of the frame. The convenience of this arrangement will be apparent on reference to the illustration.

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Fig. 223.

The hydraulic jack (Fig. 224) is a very great advance in every way on any form of screw-jack in existence. It is a simple means of raising enormous weights with very small effort, with, of course, a proportionate loss in speed. It is invaluable when heavy loads have to be moved or raised by hand labour alone, for by its use large pieces of material or machinery can be placed in their permanent positions without entailing the installation of expensive lifting apparatus, such as cranes, etc. Although the full extent of a single lift with the hydraulic jack is seldom more than 10 inches, yet, by means of properly arranged timber "packing" to support the load at the height attained by one lift, the machine or material being raised can be lifted to any desired height by successive lifting and packing. In Fig. 224 a section of the most usual form is given, which combines in itself two functions, lifting either by the head A or by the foot N, the latter arrangement being intended for use when space is limited.

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Fig. 224.

The construction of the jack is as follows: Inside the upper part a force pump H is fitted, having a plunger G which is actuated by the hand-lever D, the fulcrum of which is at c. Below the delivery valve d of this pump is the actual ram M of the jack, which slides freely in the cylinder K, the head being made water-tight in the cylinder by means of the cup-leather a; the difference in area between the ram G of the force-pump and the ram M of the jack representing the gain in pressure. On raising the lever D the water is drawn from the cistern E through the suction valve b into the barrel of the force pump. Pressing the lever downwards closes the valve b, and forces the water through the delivery valve d into the space J above the main ram. Continuing this operation, the ram M moves downwards or the cylinder K upwards, until the jack has reached its maximum travel, when the water finds its way out of the blow-hole L, automatically stopping any further relative movement between ram and cylinder.

When about to operate the jack, the cylinder K should be brought down to the bottom of the ram as shown in Fig. 224. The cistern E should then be filled with water through the hole left when the charging screw C has been removed. Care should be taken that clean water only is used in the jack, as any grit, sand, etc., contained in the water is liable to both choke the valves of the force pump and cut the cup-leather a and cause it to leak. To prevent the water freezing in cold weather it is advisable to fill the cistern with a mixture of water and glycerine in the proportion of three to one. This mixture not only serves to reduce the freezing-point, but also to keep the leather in good condition, and lubricate the gear generally. The cistern having been filled, the lowering screw F is unscrewed and the lever D worked up and down a few times, thus forcing water into the space H and driving the air, present in H previously, through the open valve e back into the cistern E, whence it can escape through the air passage B.

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Fig. 225.

This air passage should be left slightly open all the time the jack is being used, to allow air to enter and escape from the cistern. When all the air has been expelled from the space H the screw F is tightened, closing the valve e. The jack is now ready for work, and on being placed in position under the load to be raised, and the lever D being worked, the cylinder K will rise on the ram M until the blow-hole L rises above the cup-leather, after which no further motion can be obtained. It is not usual, however, to work the jack to its full limit, as the water under heavy pressure passing out of the hole L is likely to damage the cup-leather of the ram. To lower the jack the screw F is slacked back, a passage being thus provided for the water to flow back from the space J over the ram to the cistern E. If the jack is being lowered under load the speed of lowering can be controlled absolutely by adjusting the screw F and allowing the water to pass as slowly or rapidly as may be required.

With regard to lifting on the foot N, it should be noted that it is not advisable to lift more than about 50 per cent. of the load for which the jack is constructed to carry on the head A, if the distance to be travelled is more than a few inches, it being apparent from the position of the foot that the lift is not nearly so direct if it be used.

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Fig. 226.