To Thomas Jefferson February, 16th, 1789
To Thomas Jefferson February, 16th, 1789
No. 13, BROAD STREET BUILDINGS (LONDON, ENGLAND),
Your favor of the 23rd February continued to the 11th of January came safe to hand for which I thank you. I begin this without knowing of any opportunity of conveyance, and shall follow the method of your letter by writing on till an opportunity offers.
I thank you for the many and judicious observations about my bridge. I am exactly in your ideas as you will perceive by the following account. I went to the Iron Works the latter end of October; my intention at the time of writing to you was to construct an experiment arch of 250 feet, but in the first place, the season was too far advanced to work out of doors and an arch of that extent could not be worked within doors, and neatly there was a prospect of a real bridge being wanted on the spot of 90 feet extent. The person who appeared disposed to erect a bridge was Mr. Foljambe, nephew to the late Sir George Saville and member in the last Parliament for Yorkshire. He lives about three miles from the works and the river Don runs in front of his house over which there is an ill constructed bridge which he wants to remove. These circumstances determined me to begin an arch of 90 feet with an elevation of 5 feet. This extent I could manage within doors by working half the arch at a time. Having found a short wall suited to my purpose, I set off a center and five feet for the height of the arch, and forty five feet each way for the extent, then suspended a cord and left it to stretch itself for a day, then took off the ordinates at every foot (for one half the arch only). Having already calculated the ordinates of an arch of a circle of the same extent I compared them together and found scarcely any certain distinguishable difference, the reason of this is that however considerable the difference may be when the segment is a semi-circle that difference is contained between the 1st and 60th is 70 degrees reckoning from the bases of the arch, and above that the catenary appears to me to unite with the arch of the circle or exceedingly nearly thereto so that I conclude that the treatises on catenarian arches apply to the semi-circle or a very large portion of it. I annex a sketch to help out my meaning.
Having taken my measurements I transferred them to the working floor. 1st I set off half the line divided into feet; 2d the ordinates upon it; 3rd drove nails at the extremity of every ordinate; 4th bent a bar of wood over them corresponding to the swinging cord on the wall, above this first bar, and at the distance the blocks would occupy, I set off all the other bars and struck the radii thro [ugh] the whole number; which marked the places where the holes were to be cut and consequently the wooden bars became patterns for the iron bars.
I had calculated on drilling the holes for which I had allowed 8 sterling each in my private[OMITTED PICTURE HERE OF SEMI-CIRCLE] estimation, but I found, when at the works, that I could punch a square, or oblong square hole for 1 or 1 4 each. This was gratifying to me, not only because it was under my estimation, but because it took away less of the bar in breadth than a round hole of the same capacity would do, and made the work in every respect stronger and firmer. I was very unwilling to cut the bar longitudinally, and for the same reasons you mention therefore did not do it yet I was apprehensive of difficulty in getting the work together owing to diverging of the bolts, but this J think I have completely got over by putting the work together with wood bolts, and then driving them out with the iron ones.
Having made all my patterns of bars, and a pattern for my blocks, and chosen my iron 3 inches by 3/4 we began punching the holes. To do this it is necessary the iron bar be treated hot. When this was mentioned to me I pondered a little on the effects of heat, and instead of marking the iron bar when cold from the wood pattern, I first treated it and then marked and punched it, and that only one hole at a time; by this method the changes of atmospherical heat and cold are prevented operating on the bars while they are under the operation, as it is always the same season to the bar whether the season of the year be summer or winter, and as the wood patterns is laid to the bar for every fresh hole, there can be no accumulation of error, if any, would happen, and the square hole [drawing of a square here] can be corrected by a file whereas the round one could not.
A great part of our time, as you will naturally suppose was taken up in preparations, but after we began to work we went on rapidly, and that without any mistake, or anything to alter or amend. The foreman of the works is a relation to the proprietors, an excellent mechanic, and who fell into all my ideas with great ease and penetration. I stayed at the works till one half the rib, 45 feet, was completed and framed horizontally together, and came up to London at the meeting of Parliament on the 4th of December. The foreman, whom, as I told him, I should appoint “President of the Board of Works in my absence,” wrote me word that he has got the other half together with much less trouble than the first. He is now preparing for erecting, and I for returning1.
- Following this paragraph is a continuation of the letter dated February 26th, and March 12th, 1789, but since this material relates solely to political issues it has been separated from the above.