junction of two days' work is a practical impossibility, though it is materially reduced by the overlapping of the rods and the care exercised in getting clean junctions of concrete.

The concrete was mixed by means of portable rotary mixers, electrically driven and placed in such a position that the concrete passed from the mixer through chutes directly into the forms. The concrete mixture was rather wet in all cases. About 1.7 cubic feet of water was used for each two-bag batch of 1:2:4 concrete, being 14 per cent. of the volume of the aggregates measured loose, or 19 per cent. of the volume of the concrete. Using gravel instead of stone in concrete is more economical and results in a smooth finished job.

It took a gang consisting of one foreman, one engineer and seventeen laborers to mix the concrete, and one foreman and eight to ten laborers to place it.

All forms used, except for jack arches, were of wood and designed by the contractor's engineer, so that they could be removed in panels or sections and were generally used from fifteen to twenty times. For jack arches between the steel bents of stations, iron forms (No. 16 gauge metal) were used.

The winter of 1910 to 1911 was very severe, requiring the exercise of considerable care to prevent the concrete from freezing. Here, as on the Tidewater contract, too large a quantity of concrete was placed to permit of depending merely on heating sand and gravel in the congested street. Figure 122-16 shows the method employed by the E. E. Smith Co. in heating water to a temperature of 150 degrees for their mixing plant during freezing weather. Old vertical boilers were used. The water was supplied near the bottom of the boiler from the city's supply at about 40 pounds pressure. The heated water was drawn off from the top of the boiler and led to a small tank over the portable mixer. The fuel used was wood, which under ordinary circumstances was a waste. One boiler was used for each mixer, except where two mixers were run in pairs. Concrete tested in the forms usually showed a temperature of about 55 degrees. The writer's instructions were that during freezing weather no concrete should have a temperature of less than 40 degrees tested in the forms. A pair of mixers delivered about 100 cubic yards of concrete in one 8-hour shift.

Having the boilers right over the point at which the concrete was to be placed made it convenient to wash out the forms with

hot water just prior to placing concrete. Since the method of heating the water was similar to that used in the ordinary boiler attached to a kitchen range no licensed stationary engineer was required.

The water for the three contracts under discussion was obtained from the city's mains, metered and paid for at the same rate as charged to any other citizen-10 cents per 100 cubic feet. For all uses combined the cost of water was less than $50 per contract per month. This sum of money is certainly not large

enough to tempt anyone to get the water surreptitiously. It is cheaper to buy water than to steal it.

The total amount of concrete placed in 11-A-2 was 53,000 cubic yards, and in 11-A-4 was 68,000 cubic yards.

Mr. S. M. Purdy was chief engineer for the contractor and Mr. L. G. Burleigh was the superintendent.

While the engineers of the Public Service Commission kept. accurate force account, the resultant costs of work were known to but few men and the information was kept under lock and key. It would be unfair to hand over to one contractor the busi

ness secrets of another. Moreover, unless it was stated just what items were included in a cost figure, it would be misleading. A figure for cost of excavation or concrete for the whole job is no guide, since the cost varies very materially, depending on quantity and location of the work.

Figure 122-17 is interesting in showing what relation the cost of mixing, placing, forms, etc., bear to the total cost of concrete in each contract. The figures can be compared. horizontally, but

they are purposely made so that one contract cannot be compared directly with the other. It is not surprising to find that mixing and hauling should be less on the Tidewater, and that placing should be about the same in all three contracts. But that "forms" should be nearly the same is somewhat surprising and may be explained by saying that while the $25,000 spent for steel forms

brought the cost of forms far below the usual, the smaller wooden forms in the stations brought up the average. Again on the Smith contracts the wooden forms were in the form of panels so well made and smooth that they were used from fifteen to twenty times. Moreover, all timbers in the Smith cut were carefully placed for line and grade, so as to serve as braces for the forms.

Figure 122 - 18 is typical of a series of diagrams designed by the writer at the beginning of the Brooklyn work to show graphically the rate at which work of excavating, concreting, etc., should progress and the amount of work actually done, later, each month. The "criterion line" for excavation was drawn from the date of the executing of the contracts (November 9, 1909) to a date four months before the date set for the completion of the work. The "criterion line" for concrete was drawn so as to begin three months after executing the contract and end three months before completion of contract.

The criterion line for general progress (money diagram) covered a period of twenty-four months, the time fixed within which the contract was to be completed, of course. The criterion line thus established agreed closely with the work accomplished on Sections 11-A-2, 11-A-3 and 11-A-4, except the excavation for 11-A-3, which was done more rapidly than called for by our schedules. This was in keeping with the methods adopted for the prosecution of their work.

Visitors to the work were so impressed by the rapid work of the steam shovel and "the building of 40 feet of completed tunnel in one day," as they expressed it, that they imagined that no other work was being done so fast. This was true of the excavation though, unusually large quantities were being taken out in the Smith contracts. But when we compare the amount of concrete placed, we find the adjoining sections fully holding their

own.

The writer was in charge of the construction of the "Centre Street Loop," under Division Engineer G. H. Clark, from April, 1907, to September, 1908, and has compiled the rates of excavation on those contracts, comparing them with the rate of excavation on the three sections between Sackett Street and Forty-third Street, Brooklyn (Figure 122-19 and Figure 122-20). The comparison, while for different years, is for the same seasons of the year in all cases, and the conditions of work on the Delancey