about a wheelbarrowful of concrete to accumulate, when the end gate is raised and the concrete dumped into the waiting barrow and then wheeled to any desired location. At the other end of the machine the boiler and engine are located. When the machine is operated continuously the boiler requires about one-half ton of coal per day, the same man acting as engineer and fireman.

To operate it to advantage, the machine is located in the centre of the roadway and the broken stone dumped upon planks upon one side and the sand and cement on the other. The latter are carefully measured out and mixed dry in a long pile on a continuous platform. Men with shovels are stationed on each side, the number corresponding to the proportion of mortar and stone desired, and throw the material towards the back end of the shaft so that it may have the benefit of all the blades in the mixing. As the shaft revolves the mass moves forward according to the speed of the engine and the pitch of the blades. As the concrete falls into the wheelbarrow an experienced foreman or inspector can readily detect if it be not properly mixed and apply the remedy, so that in a very short time the machine will be operating successfully. No attempt is made to measure the stone, as it can be told by inspection whether sufficient mortar is present to fill thoroughly the voids, and that is all that is necessary. If too much or too little mortar is being used, the trouble is remedied by adding to or taking from the men at work on the stone as the occasion requires. This machine has a capacity of about 150 cubic yards of concrete per day when running smoothly under a capable foreman.

Another machine is called "The Portable Gravity Concrete Mixer" and consists of a short steel trough filled with numerous rows of steel pins, staggered to mix thoroughly the sand, cement, and broken stone that are to compose the concrete as they gravitate through the trough. At the upper ends of the trough the pins in the first row are spaced nearer together than the pins in the other row, in order that the stone passing the first row will go through the rest of the mixer without clogging.

The water is led from a barrel by a 13-inch hose to the spraypipe. The man at the bottom of the mixer who can best see the concrete operates the water-valve. The water from the spray-pipe strikes the mixer at about midway its length. By this arrange

ment the concrete is mixed dry in the upper half and wet in the lower half.

It is claimed for this mixer that concrete in rolling over and over on the bottom of a steel trough ten feet long, each and every stone being thrown from side to side by each row of pins, is mixed better than it is possible to mix it by hand or steam.

The trough delivers the concrete in a wheelbarrow or other receptacle, when it can be removed as desired.

It is probable that good results will be obtained by using either of these machines, and which would be the best for any particular work would have to be decided by the conditions.

The first or box machine would not be adapted to street work, as it is not easily moved and its action is not continuous. Wherever it is desired to have a special amount mixed, as, for instance, in making a cement sewer-pipe, this plan will insure the proper amount with very little waste, as all ingredients can be measured before being mixed.

By the last method it will be noticed that all material must be raised several feet above the place of delivery. This would be well adapted for concrete to be used in basements, as the material would all be naturally delivered at the street-level and must in any event be lowered to where it was to be used; or for work in trenches, or in fact under any conditions where the concrete would be needed several feet below the natural delivery of the material.

By either of these two machines the proportions of the different ingredients would probably be more accurately determined than by the second one described. But that has the advantage of being easily and quickly moved (a great desideratum in street work, especially in a narrow roadway) and is in a good position to be changed easily. Its results are certainly satisfactory when under the charge of intelligent workmen; but if operated by careless and unskilled laborers, the material would probably not be as well mixed as by either of the other machines. In other words, it requires more intelligent supervision.

As to the question whether concrete mixed by hand is better than that mixed by machine, it can be said that the product of either is good when properly made, and that incompetent workmen will spoil both. Mixing mortar and stone is hard work, and labor

ers will shirk it whenever possible; so that if proper systems are adopted for obtaining and applying the right proportions, it would seem that concrete mixed by machinery ought to give more uniform results than that mixed by hand.

In the preceding pages some examples have been given of quantities of concrete obtained from certain mixtures of cement, sand, and stone in the laboratory, so that it will be of interest to know of some of the results in actual work carried out on a large scale. It must be understood that different-sized barrels, different kinds of sand, and the varying amount of voids in the broken stone used will materially affect final results.

In making concrete for dam No. 11 on the Great Kanawha River Improvement, eleven batches, each containing 2 barrels of cement, 15 cubic feet of sand, and 33 cubic feet of broken stone, made 396 cubic feet or 143 cubic yards of concrete when rammed in place. Assuming a barrel of cement to be equal to 3.75 cubic feet, this would make the proportions by volume 1 cement, 2 sand, and 4.4 broken stone, and would give an increase of concrete over broken stone used of 9.1 per cent. The amount of material used for one yard of concrete was 1 barrels of cement, 114 cubic feet of sand, and 24 cubic feet of stone.

On a piece of work where 1000 barrels of Portland cement was used and the concrete mixed cement 1, sand 2, and 24-inch broken stone 4, the average amount obtained was 20 cubic feet per barrel of cement. The broken stone was well graded in size, and the voids, though not determined, must have been small. This would be 1.35 barrels of cement for 1 cubic yard of concrete.

On two separate occasions the author had accurate records kept on street work where the concrete was mixed by machine in the proportion of 1:2:4, and in one case 97 barrels of cement made. 81 cubic yards, and in the other 106 barrels of cement made 27 cubic yards of concrete, or almost exactly 1.20 barrels of cement per cubic yard. In these particular cases the parts of sand and stone were taken with the loose cement as a unit.

The author once laid a quantity of concrete mixed 1:2:5 in a shape and place where it was difficult to get exact measurements, and he was allowed by the engineers in charge ten per cent in excess of broken stone used.