Th: Jefferson.
LETTER CLXXVI.—TO THOMAS PAINE, December 23,1788
TO THOMAS PAINE.
Paris, December 23,1788.
Dear Sir,
It is true that I received very long ago your favors of September the 9th and 15th, and that I have been in daily intention of answering them, fully and confidentially; but you know such a correspondence between you and me cannot pass through the post, nor even by the couriers of ambassadors. The French packet-boats being discontinued, I am now obliged to watch opportunities by Americans going to London, to write my letters to America. Hence it has happened, that these, the sole opportunities by which I can write to you without fear, have been lost, by the multitude of American letters I had to write. I now determine, without foreseeing any such conveyance, to begin my letter to you, so that when a conveyance occurs, I shall only have to add recent occurrences. Notwithstanding the interval of my answer which has taken place, I must beg a continuance of your correspondence; because I have great confidence in your communications, and since Mr. Adams’s departure, I am in need of authentic information from that country.
I will begin with the subject of your bridge, in which I feel myself interested; and it is with great pleasure that I learn, by your favor of the 16th, that the execution of the arch of experiment exceeds your expectations. In your former letter you mention, that, instead of arranging your tubes and bolts as ordinates to the cord of the arch, you had reverted to your first idea, of arranging them in the direction of radii. I am sure it will gain both in beauty and strength. It is true that the divergence of these radii recurs as a difficulty, in getting the rails on upon the bolts; but I thought this fully removed by the answer you first gave me, when I suggested that difficulty, to wit, that you should place the rails first, and drive the bolts through them, and not, as I had imagined, place the bolts first, and put the rails on them. I must doubt whether what you now suggest will be as good as your first idea; to wit, to have every rail split into two pieces longitudinally, so that there shall be but the halves of the holes in each, and then to clamp the two halves together. The solidity of this method cannot be equal to that of the solid rail, and it increases the suspicious parts of the whole machine, which, in a first experiment, ought to be rendered as few as possible. But of all this the practical iron men are much better judges than we theorists. You hesitate between the catenary and portion of a circle. I have lately received from Italy a treatise on the equilibrium of arches, by the Abbe Mascheroni. 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. This interests Mr. Rumsey, who had hoped the same thing. D’Arnal’s privilege was published in a paper of the 10th of November. Probably, therefore, his application for it was previous to the delivery of Mr. Rumsey’s papers to the secretary of the Academy of Sciences, which was in the latter part of the month of August. However, D’Arnal is not a formidable competitor. He is not in circumstances to make any use himself of his privilege, and he has so illy succeeded with a steam-mill he erected at Nismes, that he is not likely to engage others to venture in his projects. To say another word of the catenarian arch, without caring about mathematical demonstrations, its nature proves it to be in equilibrio in every point. It is the arch formed by a string fixed at both ends, and swaying loose in all the intermediate points. Thus at liberty, they must finally take that position, wherein every one will be equally pressed; for if any one was more pressed than the neighboring point, it would give way, from the flexibility of the matter of the string.
I am, with sentiments of sincere esteem and attachment, Dear Sir, your friend and servant,