material on the outside, or else to employ a double-wall cofferdam filled with clay.

In relation to the comparative economics of a caisson, put down by open-dredging to a depth that is absolutely great enough to insure against disaster from scour, and a crib sunk to a reasonable depth and filled with long piles, the latter generally will be found to be much less expensive; but cases will occasionally occur when the reverse is true, hence it is well always to make complete comparative estimates of cost. These require only a few minutes' work for an experienced bridge computer.

Once in a while it occurs that the pneumatic process has to be employed for one or more piers of a long bridge, the others being sunk by open- dredging. The question then arises as to how many to put down by the more expensive process; and it should be solved more with reference to the expediting of the construction than to using pneumatics only where actually required. For instance, if all the piers but one could be handled by open-dredging, it would be found that the unit price for the base of that one would run extravagantly high, because it would have to take care of a large overhead charge for use and transportation both to and fro of the pneumatic machinery and other outfit. In that case one should figure how much it would actually cost to put down another pier by pneumatics when everything necessary is on the ground and ready to move over to the site of the next pier, then compare the result with the cost of that pier sunk by open-dredging. Generally but little difference will be found, and, therefore, it might prove truly economic to keep both outfits occupied.

The sooner any piece of bridge construction is finished, the sooner will the erection contractor be ready to undertake another contract—besides it is often the case that an expeditious handling of the work will avoid a rise of river or the advent of other unfavorable working conditions. It is, therefore, logical to conclude as a general proposition that the harder a contractor drives his work the more money will he net from his operations, even if it appear to the casual observer that lie is spending cash rather recklessly for the purpose of finishing the work quickly. Of course, if the contractor is absolutely sure that he will have no work at all to keep his force occupied after a certain job that he is on is completed, it will not pay him to spend any extra money to rush it; but, on the other hand, there is nothing to be gained by dragging it out unnecessarily. It would be better to finish it and trust to luck about getting another contract.

In foundations for trestles there sometimes arises the economic question whether, in order to obtain the requisite bearing area, it would be better to use plain concrete and go rather deep by stepping off the base in the old- fashioned way, or to spread out quickly by adopting reinforcement. The surest way to settle the question is to proportion a pedestal or two by each method and compare results. If for any reason the cost of the excavation should run high, the reinforcement method will have a decided advantage. Again, as the volume of the concrete is much smaller for that type of base