Second. In regard to the difference in the expenditures of time in climbing up and down the approaches of the two crossings under comparison, the gravity of this matter would be duly appreciated by the railroad company and more or less by the operators of trucks, but it would not be recognized by automobile owners and users.

Third. As to the agreeableness of the two kinds of crossing, while people do not particularly fancy going under ground before they are ultimately compelled to, they soon become accustomed to passing beneath the water in electric cars, as is evidenced by the many New York business men and women who reside on Long Island or in New Jersey. Perhaps in time the drivers of trucks would become so used to traversing tunnels as not to object to the gloom that is inherent in such passage; but the general public, almost to a man (and certainly to a woman), would always greatly prefer driving over a structure that provides good air and light, and usually a fine view of the harbor and the surrounding country, in comparison with traversing a long, cramped, and dingy tube.

Fourth. In relation to the question of sanitation, there is practically no greater danger to health in passing through a tunnel in which the power used is always electrical than there is in traversing a bridge; but the safe ventilation of a tube carrying automobile traffic is as yet an unsolved problem. Figures show that such ventilation, if feasible, would be exceedingly expensive, and the velocity of the passing air would be excessive.

Again, as carbon monoxide, like arsenic, is a cumulative poison, there exists a possibility that the regular daily passage through a tube where the gas remains constantly, even in minute quantities, would eventually undermine one's health. Besides, there is always the chance of a blockade of traffic with the tunnel full of automobiles, and these may be counted upon to discharge more or less products of combustion even when standing still. Such a blockade might result in a holocaust.

The author sees no serious objection, however, to building the contemplated highway tunnels under the North River; because if, after completion, they prove to be unsafe, or otherwise unsatisfactory for automobile traffic, they can either be used by electric railway cars or else a moving platform can be put in to carry vehicles through without letting them use their own power. The experiment of automobile transportation through long tunnels might be worth making, for the results in any event would prove of great interest and value to the engineering profession, as well as to the general public.

In making the comparative estimates of cost of bridges and tunnels for this crossing, the author adopted four (4) per cent grades on the approaches of both structures and clear roadways of twenty-two feet, with sidewalks eleven feet wide. He utilized as a basis for his comparison the cost estimates for tunnels given in the report of Clifford M. Holland, M. Am. Soc. C. E., Chief Engineer  of  the  New  York  State  Bridge  and  Tunnel  Commission and  the  New  Jersey  Interstate  Bridge  and  Tunnel  Commission; but