wooden-block pavement supported by reinforced-concrete slab, and the side-walks to be of reinforced granitoid.

In the electric-railway-structure comparison, the floors were assumed to be of the open type, having wooden ties and guardrails and the usual steel rails; and eight lines of track were adopted so as to compare with four lines of double-track tubes and with eight lines of single-track tubes.

In every possible manner the comparison was made fair and equitable, excepting that the costs of right-of-way and property damages for the approaches, for obvious reasons, had to be ignored. This militated against the tunnel; hence in any actual case of comparison an allowance would have to be made for the difference in costs of right-of-way and property damages for the approaches to the two structures. In the case of the bridge, by purchasing a large city-block close to the water and building a spiral approach thereon, the cost of the said right-of-way and property damages would be reduced to a minimum; and even that cost might be offset by constructing a high office building above the spiral. Such a building, owing to its location, ought to possess a high rental value.

Fig. 6a shows the total costs of main span, piers, and approaches for both highway and electric railway bridges and those of the corresponding tunnels.

As before indicated, the shortest span-length that can be used for the proposed North River bridge is about 2,900 feet, for which length the diagram gives approximately the following costs:

This indicates that there is a saving of cost in favor of the highway bridge amounting to $16,500,000, and one of $4,500,000, in favor of single-track, electric-railway tunnels. The former saving is far greater than the difference in costs of right-of-way and property damages for bridge and tunnel is ever likely to be; hence the conclusion is reached that, for the proposed North River crossing, it is not only better from every point of view, but also more economic to carry automobile traffic by a bridge and electric trains by single-track tunnels.

The question arises as to how cheaply there could be built at present prices a combination of bridge and tunnels to care for both kinds of traffic. For a number of years two single-track tubes would take care of the electric-railway trains, hence the cost would be as follows: