Taylor's Mixer. A later adaptation of the same type of mixer has the revolving chamber in the form of a double cone (fig. 30).

"In mixing concrete the materials are filled into the measuring hopper in the usual proportions; the sliding door is then withdrawn and they are admitted into the revolving mixing cones, to mix dry first; water is then supplied through the hollow supporting shaft.

"A few revolutions, say 15, serve to thoroughly mix the materials, and the delivery doors, which are closed perfectly tight while the mixing is proceeding, being simultaneously opened by the lever and clutch, the contents are dropped through a shoot into suitable trucks or skips, or directly on to the work in hand.

"A one-yard mixer can produce, in ordinary working, 24 cube yards of the very best concrete in one hour at a cost for labour of about 4d. per yard. If fitted with a steam hoist and special service trucks an output of 30 yards per hour of thoroughly well mixed concrete can be guaranteed, the cost being greatly reduced. The machines are made either stationary or portable, and of capacities varying fromto 1 cube yards" (Manufacturers' Circular).

The machines can also be worked by gas engine or electric motor. They are supplied by Messrs. Henry Balfour & Co., Ltd., of Leven, Fife.

The Taylor mixer has been employed at the Keyham Dockyard extension works, at Barry Dock, at Methil Dock extension, at Seaham Harbour, &c.

Continuous Mixers Carey-Latham Mixer.-In this machine the sand and ballast are supplied systematically, by means of ascending buckets, to the mixing cylinder (figs. 31 and 32), where they are met by a supply of cement, the quantity of which is regulated by an archimedean screw. The process can thus be carried on uninterruptedly for any length of time.

Incorporation is "carried out in a revolving cylinder in which are fitted inclined blades or vanes, which lift and tumble the materials some 50 times before delivery, first in the dry and afterwards in the wet state. During this process the blades or vanes, which are carried from a central shaft, revolve with the cylinder in the same direction, but at a slightly less speed, whereby they are constantly changing their position, acting as scrapers, and thus prevent the setting of the cement on the blades and inner surface of the mixing cylinder. The water required for the concrete passes through the central shaft, and is sprayed out on the materials as they are tumbled about in the mixer."

The machines are manufactured by Messrs. John H. Wilson & Co., Ltd., of Liverpool, in sizes capable of discharging from 10 to 30 cubic yards per hour.

The Carey-Latham mixer has been used in connection with dock and harbour works at Peterhead, Newhaven, Sydney, Hong Kong, Yokohama, Odessa, Bilbao, New York, &c.

Sutcliffe Mixer. The principle of this machine (fig. 33) is embodied in the method adopted for measuring the quantities of material. The cement is discharged into the uppermost hopper (fig. 34), the floor of which is a cylinder with three grooves of equal area and capacity in its surface. The cylinder is turned by a hand wheel, and an angular displacement of 60° causes the contents of one of the grooves to be emptied into the lower hopper where it meets with the proper supply of gravel. The gravel is discharged from hand barrows, and the cement grooves are so regulated that one grooveful corresponds to a barrow load. When the lower hopper is full, the contents are allowed to fall through three trap doors, opened consecutively, on to a moving band which conveys the dry materials to a

Cement
Hopper

Gravel and

Sand Hopper

Conveying Band

Mixing

Vanes

Fig. 33.--Sutcliffe Concrete Mixer-Elevation.

series of trays at the top of a shoot, water being added from a supply pipe at the level of the topmost tray. Each side of the machine is symmetrical, and, by means of an oscillating vane for the deflection of the cement supply, the machine becomes double acting, so that there is absolutely no break in the discharge, which takes place from each side of the lower hopper alternately.

This machine has been very extensively used at the Liverpool Docks for a number of years. It has proved capable of turning out over 300 cubic yards of concrete in a working day of ten hours, but the normal rate of supply lies between 200 and 300 cubic yards per day.

Gravity Mixer.-An American machine in which there are no moving parts, the whole process of mixing being performed by numerous rows of pins, which intercept and sift the material during its descent through a shoot, is effectively illustrated in fig. 35. The ingredients are first deposited in measured quantities upon the platform, and then shovelled by hand to

the shoot, at the top of which the water supply is added. The concrete, however, does not actually become wetted until it reaches the fifth row of pins, the process prior to this being that of dry mixing. A gate, or valve, at the foot of the shoot, regulates the discharge if not required to be continuous.

The machine has been used in England at the Liverpool Docks, the London and India Docks, and at Chatham Dockyard. At the Canada Branch Dock (No. 2 contract), Liverpool, it proved capable of discharging rather less than 100 cubic yards per day when fed continuously. This represents a much more moderate output than those of the machines previously described, but the concrete was of a very satisfactory quality. The advantages possessed by a mixer of this type, when used in undertakings not necessitating a rapid supply, are lightness, mobility, and economy.

Concrete Moulds.-Closely connected with concrete mixers are the temporary wooden moulds within which the fluid concrete is deposited.

For the back of a dock or quay wall, which is usually designed in a series of horizontal offsets, ordinary deals on edge form a sufficient and satisfactory enclosure. The offsets are rarely more than a few planks in height; these are stiffened by short verticals at the back, and the whole rests upon the ledge next below. The same method, with a slight modification, may be adopted for the altar courses of a graving dock. The profile is rounded and the face of the mould carefully planed (fig. 36).

For the front of a dock or quay wall, the moulds are prepared in uniform sizes of any convenient dimensions. Two methods of supporting them are illustrated.

In the first case the moulds (fig. 37) rest upon short cantilevers projecting from the wall. These cantilevers (fig. 38) are of timber, about 4 by 3 inches section, with their ends laid upon the previously completed work