bottom, and twenty-four inches deep. The brick side-walls (A) twelve inches thick at top, and built vertically on the interior and outside, rested on an annular trench of concrete,

Fig. 14. Section through the axis of the Fort Warren Mortar Mill.

A, Annular trough for mixing the mortar.

A', Brick sides of trough.

B, Central brick cylinder.

C, Annular space for holding lime in paste.

one foot thick, which was laid on an annular bed of broken stone, two feet thick, for drainage.

In the centre of the circle enclosed by the trough, a vertical post, surrounded with broken stone, encased by a brick cylinder (B) has a gudgeon at top, around which the horizontal shaft (E) turns, that gives motion to the wheel (D) for mixing the mortar.

The wheel (D) is made of wood on the sides and periphery, and has an iron tire twelve inches broad and half an inch thick; the interior being filled with sand to give it sufficient weight to grind any lumps in the lime to a paste. The diameter of the wheel is eight feet, and thickness eight inches.

The radius of the horse track for working the wheel is twenty feet.

The annular space between the trough and the brick cylinder in the centre is floored with concrete, resting on a bed of broken stone.

Lieut. W. H. Wright, in his Treatise on Mortars, thus describes the use made of this annular ring: "The space between the cylinder and trough is used as a reservoir for the slaked lime. It is conveniently divided by means of movable radial partitions into sixteen equal parts," each containing the sixteenth part of a cask of lime in paste.

A wooden trough (F) leads from the reservoir where the

lime is slaked and converted into a creamy consistence, to the annular ring (C), where it is allowed to stand as long as possible before being thrown, with the requisite quantity of sand, into the mill.

The malaxator.-Many advantages are claimed for a mill designed by M. Coignet, recently introduced in France, and employed in mixing béton aggloméré for the works in and about Paris. It is called a malaxator, and consists of twin screws, having their helices interlocked, and turning and exerting their force in the same direction. This machine may be described as follows:

A is the frame of the machine, having at the upper end the cross-pieces B, upon which are mounted the gearings, and at the lower part the cross-piece cc', upon which are fixed the rests or steps for the lower part of the helices to run in.

D are the cores of the helices, upon which are fastened either continuous or interrupted blades S S S, forming the thread of the helix. Continuous blades are more generally used.

K are wagon-wheels, mounted on an axle, which enable the machine to be transported thereon, and which, when the machine is in use, serve to maintain the malaxator at its proper

inclination (about twenty-five degrees). The brace J is used to steady the malaxator.

M Nm N', gearings of any kind for giving motion to the helices, either by steam, horse-power, or hand-power; q, conical sleeves or stoppers, adjustable upon the shafts D, for regulating the exodus of the artificial stone paste, and by retarding the same, increasing the pressure and malaxation of the paste in the part Q' of the machine.

Q, body of the malaxator, corresponding in shape and size to the helices.

P, receiving chamber, where the materials enter the mal

axator.

T, sand hopper, with its adjustable register or gate t, and, when required, a sifting apparatus; q', sliding gate, to allow of the drainage of the machine.

S'S', feeding screws, working in the lower part of the two hoppers R' R', the one for lime, the other for sand, or any other material or substance to be introduced into the artificial stone paste, and feeding the same to the chamber P; r", pulleys, for chains or belts g, for transmitting the movement to the feeding screws S' S'; t' t', spur-wheel and pinion (changeable for others of different relative speed), for regulating the exact amount of the two substances in the hoppers R' R', to be delivered, in so many turns of the helices, into the receiving chamber P.

Z is a pipe for supplying the water, for which there is an overflow at W. The sand being drowned or fully saturated in a given proportion, by varying the overflow W, gives the proper amount of water for each turn of the helices.

Hare movable wooden shafts, which are placed in proper straps in the machine, and serve to hitch or harness a horse to the same when it has to be taken from one place to another, making it a perfect wagon.

The advantages claimed for the malaxator are the following: First. The apparatus, having the receiving chamber P upon the ground, is fed easily, with little labor; and the part Q', or delivery, being elevated, allows of a wheelbarrow or basket being placed under to receive the artificial stone paste. This inclination also causes a more powerful malaxation, by retarding the progress of the matter, owing to the specific gravity. Second. The gearings are out of the way, away from sand, water, dust, etc.

Third. The helices having their blades interlaid, their action upon the materials is of quite a different character than when said helices are not thus conjugated.

Fourth. The sand is gauged by a register. The lime and the hydraulic cement, the coloring matter, texture giver, or any other material used, may be also fed automically, and the machine once set by the inspector, the product is invariably the same, besides saving the labor of a hand whose trustworthiness is required to obtain good results. The continuous introduction by small and regular quantities of the different

substances, and the constant amount of the water supplied to the sand, place the materials in the best circumstances for producing, by proper action of the helices, an excellent result,

difficult to obtain if the component ingredients had been thrown in by shovel or basketfuls at a time. (See Professional Papers, Corps of Engineers, No. 19).

Another form of mill, which is shown in Fig. 16, has been made use of in France for mixing certain kinds of beton. It consists of a vertical cylinder a resting on a cylindrical base of cast iron b. A vertical shaft c passes through the cylinder, having attached to it curved arins d, which, by revolving horizontally, serve to mix the sand and lime. The distributor Qrevolves horizontally, receives the sand and lime which come from the conducting trough I, and distributes them equally around for mixing. Short stationary arms E E are attached to the side of the cylinder, and form, with the movable arms, breaks for dashing and mixing the sand and lime. Three helicoidal blades e e, attached to the lower part of the shaft, force the mixture downwards and outwards. Cycloidal arms ff, revolving horizontally near the floor of the cylinder, expel the mixture at the side opening around the bottom. A movable band of iron G G, by being moved up or down, enlarges or diminishes the opening around the bottom. h h, vertical guiding shafts for movable band. HH, handles by which the band G G is moved. A plate N is attached to c and revolves horizontally, receiving the mixture from the cylinder. A curved plate of iron L, fixed to immovable bottom-plate P, scrapes mixture from N as it revolves.

143. Setting and durability of mortars. Mortar of common lime, without any addition of puzzolana, will not set in humid situations, like the foundations of edifices, until after a very long lapse of time. They set very soon when exposed to the air, or to an atmosphere of carbonic acid gas. If, after having become hard in the open air, they are placed under water, they in time lose their cohesion and fall to pieces.

144. Common mortars, which have had time to harden, resist the action of severe frosts very well, if they are made rather poor, or with an excess of sand. The sand should be over 2.40 parts, in bulk, to one volume of the lime in paste; and coarse sand is found to give better results than fine sand.

145. Good hydraulic mortars set equally well in damp situations, and in the open air; and those which have hardened in the air will retain their hardness when immersed in water. They also resist well the action of frost, if they have had time to set before exposure to it; but, like common mortars, they require to be made with an excess of sand, to withstand well atmospheric changes.

146. The surface of a mass of hydraulic mortar, whether