It
appears to be a very scientifical work. I have not yet had time to
engage in it; but I find that the conclusions of his demonstrations are,
that every part of the catenary is in perfect equilibrium. It is a great
point, then, in a new experiment, to adopt the sole arch, where the
pressure will be equally borne by every point of it. If any one point is
pushed with accumulated pressure, it will introduce a danger, foreign to
the essential part of the plan. The difficulty you suggest, is, that the
rails being all in catenaries, the tubes must be of different lengths,
as these approach nearer or recede farther from each other, and
therefore you recur to the portions of concentric circles, which are
equidistant in all their parts. But I would rather propose, that you
make your middle rail an exact catenary, and the interior and exterior
rails parallels to that. It is true, they will not be exact catenaries,
but they will depart very little from it; much less than portions of
circles will. Nothing has been done here on the subject since you went
away. There is an Abbe D'Arnal at Nismes, who had obtained an exclusive
privilege for navigating the rivers of this country by the aid of the
steam-engine.
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