machines, the steam screw and wheel-gear worked cranes, screw and wheel-gear hoists, as well as the screw press for making up bales of goods mentioned in Lecture XV. For with it, you can bring to bear a force of a few pounds on the square inch or as many tons, by merely turning the handle of a small cock, and with a certainty of action unattainable by any other means.

The Hydraulic Jack is a combined hydraulic press and force pump, arranged in such a compact form as to be readily portable, and applied to lifting heavy weights through short distances. It therefore effects the same objects as the screw-jack described in Lecture XV., but with less manual effort or greater mechanical advantage.

The base on which the jack rests is continued upwards in the form of a cylindrical plunger, so as to constitute the ram of the hydraulic cylinder HC. Along one side of this ram there is cut a grooved parallel guide slot GS, into which fits a steel set pin, screwed through the centre of a nipple cast on the side of the cylinder (not shown in the drawings) for the purpose of guiding the latter up and down without allowing it to turn round. The top of the ram is then bolted with a water-tight cup leather CL, by means of a large washer and screw-bolt.

The action of this cup leather is precisely the same as the leather collar in the cylinder of the Bramah press already described; but it has only to be pressed by the water in one direction -viz., against the sides of the truly-bored cast-steel cylinder, instead of against both the ram and the cylinder neck, as in the previous case. The head H and upper portion of the machine is of square section, and is screwed on to the hydraulic cylinder in the manner shown by the figure. It contains a water reservoir WR, which may be filled or emptied through a small hole by taking out the screw-plug SP.* In the centre line of the head

This screw plug SP is slackened back a little to let the air in or out

piece there is placed a small force pump, the lower end of which is screwed into the centre of the upper end of the hydraulic cylinder. This pump is worked by the up-and-down movement of a handle placed on the squared outstanding end of the turned crank shaft CS. To the centre of the crank shaft there is fixed a crank C, which gears with a slot in the force-pump plunger P, and thus the motion of the handle is communicated to the pump plunger in a reduced amount, corresponding to the inverse ratio of the lengths of the handle and the crank from the fulcrum or centre of the crank shaft. By comparing the right-hand section of the water reservoir, and the section on the line AB, with the vertical left-hand section of the jack, it will be seen where the inlet and delivery valves IV and DV are situated. On raising the pump plunger P, water is drawn from WR into the lower end of the pump barrel through IV, and on depressing the plunger this water is forced through the delivery valve DV into the hydraulic cylinder, thus causing a pressure between the upper ends of the cylinder and the ram, and thereby forcing the cylinder, with its grooved head H, and footstep S, upwards, and elevating whatever load may have been placed thereon. Both the inlet and outlet valves are of the kind known as "mitre valves." They have a chamfer cut on one or more parts of their turned spindles, so as to let the water in and out along these channels. . The valves are assisted in their closing action by small spiral springs SS, bearing in small cups or hollow centres, as shown more clearly in the case of DV by the enlarged section on AB.

Weems' Compound Screw and Hydraulic Jack. This is a jack combining some of the advantages of the ordinary screw-jack with those of the hydraulic one. It is often desirable to be able to bring the head or footstep into trial contact with the load before applying the water pressure. This can easily be done by turning the nut at the foot of the screw, cut on the ram of the jack. The arrangement will at once be understood from the figure. It will be observed that the load may also be lowered by turning this nut, or by the screw-tap which permits water to flow from the cylinder back into the cistern, as in the previous case. The bottom nut may be screwed hard up to the foot of the hydraulic cylinder, so as to sustain the whole load, and thus prevent overhauling through leakage of the water.

When it is necessary to lower the load or the head of the jack,

of the top of the water reservoir when working the jack. There is generally another and separate screw plug opening (as will be seen by the following figure of Weems' patent jack) for filling or emptying the water reservoir, quite independent of the above-mentioned one, which is used in this case for both purposes.

the relief valve or lowering screw, is turned so as to permit the water to escape from the hydraulic cylinder

back into the water reservoir, as clearly shown by the drawing. This may be done very gently by simply giving this screw a very small part of a complete turn; in other words, by throttling the passage between the hydraulic cylinder and the water reservoir. Or it may be done quickly by turning it through one or more revolutions. This passage can then be closed by screwing the plug home on its seat.

Mr. Croydon Marks, in his book on "Hydraulic Machinery," illustrates and describes another method of lowering the jack-head (first introduced by Mr. Butters, of the Royal Arsenal, Woolwich), where, by a particular arrangement, the inlet and delivery valves are acted upon by an extra depression of the handle, and consequent movement of the pump plunger. He also gives the main dimensions, with a drawing, of the standard 4-ton pattern as used by the British Government, where the ram has a diameter D=2", the pump plunger a diameter d=1"; and the ratio of the leverage of the handle to the crank is 16 to 1. Therefore from the previous formula we find that,

The Theoretical Advantage=

W AF D2 16 22 64
PBF d2
x=1

X

And he instances two trials by Mr. W. Anderson, the Inspectorgeneral of Ordnance Factories, to determine the efficiency of these jacks, where, with a pressure on the end of the working handle of 76 lbs., the theoretical load should have been 76 lbs. x theoretical advantage 76 x 64 = 4864 lbs., instead of which it was only 3738 lbs. ;

=

In a second trial, a load of 1064 lbs. required a pressure of 22 lbs. on the handle, and consequently the efficiency at this lighter load, as might be expected, was less, or only 74%.

EXAMPLE III.-With a hydraulic jack of the dimensions given above, and of 77% efficiency, it is desired to lift a load of 4 tons; what force must be applied to the lever handle? ANSWER.-By the previous theoretical formula,

But the efficiency of the machine is only 77%: consequently 140 lbs. is 77 per cent. of the force required—

The Hydraulic Bear, or Portable Punching Machine.This is another very useful application of the hydraulic press and force pump. It is used in every iron or steel shipbuilding-yard and bridge-building works. By comparing the drawing with the index to parts, and taking into consideration the fact that its construction and action are so very similar to the hydraulic jack already described in full detail, we need say nothing more than direct the student's attention to the action of the raising cam, and to the means by which the apparatus is lifted and suspended. In order to raise the punch for the admittance of a plate between it and the die D, the relief valve RV must first be turned backwards, and the lever L depressed. This causes the corner of the raising cam RC to force the hydraulic ram HR upwards, and the water from the hydraulic cylinder HC back into the water