The Writings of Thomas Jefferson, Vol. 6 (of 9). Томас Джефферсон

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СКАЧАТЬ adapted to the length of the pendulum of 45°, according to the most approved estimates, including those of the French commissioners. This would serve to introduce the subject to the foreign societies, in the way before proposed, reserving to ourselves the charge of communicating to them a more perfect one, when that shall have been completed.

      We may even go a step further, and make a general table of the measures, weights and coins of all nations, taking their value hypothetically for the present, from the tables in the commercial dictionary of the encyclopedia methodique, which are very extensive, and have the appearance of being made with great labor and exactness. To these I expect we must in the end recur, as a supplement for the measures which we may fail to obtain from other countries directly. Their reference is to the foot or inch of Paris, as a standard, which we may convert into parts of the second pendulum of 45°.

      I have thus, my dear sir, committed to writing my general ideas on this subject, the more freely as they are intended merely as suggestions for consideration. It is not probable they offer anything which would not have occurred to the committee itself. My apology on offering them must be found in your request. My confidence in the committee, of which I take for granted you are one, is too entire to have intruded a single idea but on that ground.

      Be assured of my affectionate and high esteem and respect.

      TO DOCTOR ROBERT PATTERSON

Monticello, November 10, 1811.

      Dear Sir,—I write this letter separate, because you may perhaps think something in the other of the same date, worth communicating to the committee.

      I accept, willingly, Mr. Voigt's offer to make me a time-piece, and with the kind of pendulum he proposes. I wish it to be as good as hands can make it, in everything useful, but no unnecessary labor to be spent on mere ornament. A plain but neat mahogany case will be preferred.

      I have a curiosity to try the length of the pendulum vibrating seconds here, and would wish Mr. Voigt to prepare one which could be substituted for that of the clock occasionally, without requiring anything more than unhanging the one and hanging the other in its place. The bob should be spherical, of lead, and its radius, I presume, about one inch. As I should not have the convenience of a room of uniform temperature, the suspending rod should be such as not to be affected by heat or cold, nor yet so heavy as to effect too sensibly the centre of oscillation. Would not a rod of wood not larger than a large wire, answer this double view? I remember Mr. Rittenhouse told me he had made experiments on some occasion, on the expansibility of wood lengthwise by heat, which satisfied him it was as good as the gridiron for a suspender of the bob. By the experiments on the strength of wood and iron in supporting weights appended to them, iron has been found but about six times as strong as wood, while its specific gravity is eight times as great. Consequently, a rod of it of equal strength, will weigh but three-fourths of one of iron, and disturb the centre of oscillation less in proportion. A rod of wood of white oak, e. g. not larger than a seine twine, would probably support a spherical bob of lead of one inch radius. It might be worked down to that size I suppose, by the cabinet-makers, who are in the practice of preparing smaller threads of wood for inlaying. The difficulty would be in making it fast to the bob at one end, and scapement at the other, so as to regulate the length with ease and accuracy. This Mr. Voigt's ingenuity can supply, and in all things I would submit the whole matter to your direction to him, and be thankful to you to give it. Yours affectionately.

      TO MR. H. A. S. DEARBORNE

Monticello, November 15, 1811.

      Sir,—Your favor of October 14 was duly received, and with it Mr. Bowditch's observations on the comet, for which I pray you to accept my thanks, and be so good as to present them to Mr. Bowditch also. I am much pleased to find that we have so able a person engaged in observing the path of this great phenomenon; and hope that from his observations and those of others of our philosophical citizens, on its orbit, we shall have ascertained, on this side of the Atlantic, whether it be one of those which have heretofore visited us. On the other side of the water they have great advantages in their well-established observatories, the magnificent instruments provided for them, and the leisure and information of their scientific men. The acquirements of Mr. Bowditch in solitude and unaided by these advantages, do him great honor.

      With respect to the eclipse of September 17. I know of no observations made in this State but my own, although I had no doubt that others had observed it. I used myself an equatorial telescope, and was aided by a friend who happened to be with me, and observed through an achromatic telescope of Dollard's. Two others attended the time-pieces. I had a perfect observation of the passage of the sun over the meridian, and the eclipse commencing but a few minutes after, left little room for error in our time. This little was corrected by the known rate of going of the clock. But we as good as lost the first appulse by a want of sufficiently early attention to be at our places, and composed. I have no confidence, therefore, by several seconds, in the time noted. The last oscillation of the two luminaries was better observed. Yet even there was a certain term of uncertainty as to the precise moment at which the indenture on the limb of the sun entirely vanished. It is therefore the forming of the annulus, and its breaking, which alone possess my entire and complete confidence. I am certain there was not an error of an instant of time in the observation of either of them. Their result therefore should not be suffered to be affected by either of the others. The four observations were as follows:

      I have thus given you, Sir, my observations, with a candid statement of their imperfections. If they can be of any use to Mr. Bowditch, it will be more than was in view when they were made; and should I hear of any other observations made in this State, I shall not fail to procure and send him a copy of them. Be so good as to present me affectionately to your much-esteemed father, and to accept the tender of my respect.

      TO MELATIAH NASH

Monticello, November 15, 1811.

      Sir,—I duly received your letter of October 24 on the publication of an Ephemeris. I have long thought it desirable that something of that kind should be published in the United States, holding a middle station between the nautical and the common popular almanacs. It would certainly be acceptable to a numerous and respectable description of our fellow citizens, who, without undertaking the higher astronomical operations, for which the former is calculated, yet occasionally wish for information beyond the scope of the common almanacs. What you propose to insert in your Ephemeris is very well so far. But I think you might give it more of the character desired by the addition of some other articles, which would not enlarge it more than a leaf or two. For instance, the equation of time is essential to the regulation of our clocks and watches, and would only add a narrow column to your 2d page. The sun's declination is often desirable, and would add but another narrow column to the same page. This last would be the more useful as an element for obtaining the rising and setting of the sun, in every part of the United States; for your Ephemeris will, I suppose, give it only for a particular parallel, as of New York, which would in a great measure restrain its circulation to that parallel. But the sun's declination would enable every one to calculate sunrise for himself, with scarcely more trouble than taking it from an Almanac. If you would add at the end of the work a formula for that calculation, as, for example, that for Delalande, § 1026, a little altered. Thus, to the Logarithmic tangent of the latitude (a constant number) add the Log. tangent of the sun's declination; taking 10 from the Index, the remainder is the line of an arch which, turned into time and added to 6 hours, gives sunrise for the winter half and sunset for the summer half of the year, to which may be added 3 lines only from the table of refractions, § 1028, or, to save even this trouble, and give the calculation ready made for every parallel, print a table of semi-diurnal arches, ranging the latitudes from 35° to 45° in a line at top and the degrees of declination in a vertical line on the left, and stating, in the line of the declination, the semi-diurnal arch for each degree of latitude, so that every one knowing the latitude of his place and the declination of the day, would find his sunrise СКАЧАТЬ