The contrasting of a bridge with a tunnel or a combination of tunnels for any proposed crossing, in respect to the question of economics, is by no means easy; because, in order to make a perfectly just comparison, the facilities afforded by the two structures should be alike. Generally, laymen make the mistake of pitting a single-track tunnel against either a double-track bridge or a purely railway tunnel against a combined-railway-and-highway bridge. As it is usually uneconomic to make the interior diameter of a tunnel tube much greater than twenty feet, because the cost of such a tube increases very rapidly with the diameter, and as any projected bridge that is in competition with a tunnel is necessarily a structure of some importance, and, therefore, wide of deck, it is evident, that the comparison will probably always be made between one bridge and two or more tunnels. Again, as the travel in a tube of twenty-foot internal diameter must either be in one direction only or else very slow, two tubes will be required in order to obtain rapid transit. The reason for this is that, in a tube just large enough for two lines of traffic in opposite directions, the speed is absolutely limited to that of the most-slowly-moving vehicle; because it would be impracticable for a rapidly-moving automobile to turn out so as to pass a slow vehicle without facing the line of traffic coming in the opposite direction; and a single breakdown would quickly block all motion. Anyone who gives the subject any thought must quickly arrive at the conclusion that the motion of traffic in any highway-tunnel tube must be restricted to one direction only.

Another obstacle to the satisfactory comparison of these fundamentally-different types of transportation structures is the as-yet-unsolved problem of ventilation. Many engineers contend that it is absolutely unsafe to run automobiles through a long tunnel because of the exceedingly-poisonous carbon-monoxide given forth during combustion; while others say they are certain that such traffic can be maintained without danger. Probably this question will be settled finally during the next five years, and in the only convincing manner possible, viz., by building a long tunnel for automobile travel and operating it. As it is stated by medical men of high authority that the action of carbon-monoxide upon the human system is cumulative, it may prove difficult, expensive, or even totally impracticable to dilute the poison to such an extent as to make the atmos-

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