but such an idea is a fallacy, because the incoming and the outgoing vehicles could enter and leave the bridge at points two blocks apart, and a double-track, electric-railway line could enter and leave by the street between. If this arrangement would not separate the incoming and the outgoing traffic sufficiently, the entrances and exits might be located four, six, or eight blocks apart—in fact there need be no restriction as to the total width of deck in case that this method of traffic separation be adopted.

By employing a spiral approach of large diameter, the traffic could leave the periphery thereof at three, or even more, points, thus making the approaches to the said spiral of different lengths, but all comparatively short.

Some months after the preceding was written with the intention of considering the treatment of the subject as closed, the author had occasion to prepare for the American Society of Civil Engineers a paper entitled "Bridge versus Tunnel for the Proposed Hudson River Crossing at New York City"; and as it gives much additional information upon the general economic question involved in this chapter, it is here reproduced practically verbatim:

Whilst making lately some extensive calculations concerning the costs and economics of long-span suspension-bridges for his forthcoming treatise on "Economics of Bridgework," the author has had occasion to figure weights of metal for a number of such spans; and by means of the resulting data he was able to undertake an investigation of the comparative costs and efficiencies of bridges and tunnels for the long-talked-of crossing of the North River at New York City. Thinking that the present is an auspicious time for a thorough discussion of the subject, he has collected and condensed the results of his labors and incorporated them in this memoir for the Society.

For some years he has been of the opinion that the best and most economic solution of the problem under consideration is to carry all street cars and subway cars beneath the water and the strictly-highway traffic above it. As far as the question of desirability is concerned, this arrangement would be the best practicable for the following reasons:

First. In respect to cost of operation, the tunnel would require a dip of ninety feet below high-water, and the bridge a rise of one hundred and eighty feet above it; consequently it is evident that, as far as the matter of expenditure of energy is concerned, the tunnel would be decidedly preferable. The difference in cost of power would be very apparent to the management of the electric railways, and possibly also to the operators of heavy trucks, but it would not be noticed at all by the owners of automobiles used mainly for pleasure traffic. When an automobilist is about to climb a long, heavy grade, he seldom thinks anything concerning how much extra his gasoline is going to cost him; but the officers of an electric railway line generally figure with the greatest of care on the item of power expense, and aim to reduce it to a minimum.