For the "Medium Condition of Market," the price per cubic yard of the shafts is to be modified by the addition or subtraction of fifteen cents for each foot of variation from the assumed average of twenty, the greater the thickness the smaller the unit price. For instance, if a shaft were 12 feet wide under coping and 18 feet wide at the bottom, the average width would be 15 feet and the unit price for medium market $12.75.

For the same market condition the unit price for mass of caissons is to be modified by the addition or subtraction of ten cents for each foot of variation from the assumed average of thirty, the wider the caisson the smaller the price per cubic yard. Again, for the said market condition, the unit price for mass of caissons is to be modified by the addition or subtraction of two cents for each foot of variation from the assumed average height of one hundred and fifty feet, the deeper the caisson the smaller the unit price. For instance, with medium condition of market, the unit price for a caisson twenty-six feet wide and two hundred and forty feet high would be

20.00 + 4 X 0.10 - 90 X 0.02 = $18.60.

For the other two assumed conditions of the market, these figures of modification would have to be multiplied by the ratios indicated in the table, viz., 0.75 and 1.25.

Without these modifications of unit prices for substructure, the investigation would be not only illogical, but incorrect. The variation in cost of shafts per cubic yard is due primarily to the lower unit cost of forms for thick piers, but also somewhat to the economy effected by manufacturing and handling larger masses of concrete. The latter reason applies also to the two variations in the cost of mass of caissons; but the main cause thereof is that the total cost of cutting edge, shelter against current, and flotation to final location are the same for a shallow base as for a deep one.

The prices per cubic yard for caissons sunk by the pneumatic process, under medium-market conditions, have been made two dollars greater than those for caissons sunk by open-dredging. This is in conformity with the author's bridge experience of nearly four decades. It is due primarily to the more rapid sinking by open dredging and the greater cost of the pneumatic outfit, but also to the fact that the pneumatic caissons are generally filled solid, while the open-dredging caissons often have their excavating wells only partially filled.

The price used for nickel steel superstructure in place for medium market conditions has been taken as eight and a half cents per pound; for the reason that the last ante-bellum figures quoted to the author made the price of nickel-steel two and a half cents per pound higher than that of carbon-steel. The weights of metal in nickel-steel superstructures were computed by means of ratios determined from diagrams given in the author's paper "Nickel Steel for Bridges."*

* See Trans. Am. Soc. Civ. Engrs. for 1909.