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David K. Hopkins to Thomas Jefferson, 3 November 1809

From David K. Hopkins

Cynepuxent 09. nov. 3d


Since I wrote you last year, I have not been Idle, I understand the Subject on which I wrote much better.—I observed1 to you that all kinds of motion aught to be communicated by the Screw or, circularly inclined plane.—with water I wou’d (for to communicate motion to a Mill) inclose a circularly inclined plane, or Screw, nearly equal in length to the fall of water, erect it perpendicular & this wou’d be the Spindle of the Mill; then we’ll Suppose there’s no opposition, the Screw wou’d yeald to the water, in consequence of which, the water wou’d fall perfectly perpendicular through2 the inclos’d Screw.—by wasteing the Same quantity of water, that is wasted in any other way to communicate motion to a Mill, there’s more power gain’d, but I cannot Say how much. we know in the overshot wheel, that nearly all the water is liberated by the time it falls half way down; much the greater part of water falls out of the buckets at that distance: but in this way not a drop escapes untill it descends to the bottom of the Screw, it runs in quietly & escapes quietly, having communicated the3 whole of its motion to the Screw in falling: that is if the Screw is opposed by grinding, if there’s no opposition it falls as rash through the Screw as tho, t’were not there.—I have never tried the experiment with a Screw longer than about a foot; you perceive that by carrying the Screw or thread round oftener, the motion will be increas’d, but the power lessen’d in a revolution & the contrary.—being at New-haven not long Since I was informed, I think by Col. Humphries, that a young man near there had communicated motion to a Mill by means of the archemedian Screw, (it was made about eighteen inches high & the water introduced below) with Success; this of Course was attended with more expence, & friction than the plan I propose the greatest difficulty with me is, whether or not there be the Same objection to the plan I propose that there is to Barkers Mill, that the water taking a centrifugal force looses its tendency downwards & causes the mill to run irregular. I cannot See how the water can either take, or, be inclined a centrifugal force in the method I propose; on this I wish your opinion most particularly as I Cannot ascertain without going to the expence of erecting one.—I remarked to you in my letter last year that the Sails of my wind-mill were on that principle. when the Mill was first put up the Sails were made in the usual Shape, I altered them a while before I wrote you last year, each Sail is a Section of a Screw, & I find about Eighty yards is as powerfull as a hundred & ten was. the Sails extend about eleven feet from the Shaft & I believe that a wind-Mill Can be built but twelve feet high to be better & Safer than when they are higher.—The observations on the action of the water on the Screw, may be applied to the action of the wind on the Sail of a wind-mill.—a Sail moveing in a horizontal direction Aught to be of the Same angle at top & bottom, but a Sail moveing in a circular direction, the Idea is foreign to what is correct to Suppose that the angle Augt to be the Same at any two distances from the Centre. The greater the distance of any pairt of the Sail from the Centre of motion, the greater is the motion of that part. a body of air rushes equally on all parts, the air cannot pass by as4 rapid near the Centre as at a more remote distance. if the Sail be a Screw it’s equally rapid & has the Same effect on a yard of Canvass near the Centre, that it has on one more remote, or, a yard of canvass does as much Service next the centre as one more remote.—

I am affraid I Cannot easily communicate5 the method Which I make use of to cause motion by application of wheels on the Screw I have applied it to a grind Stone & two hand-mills it Multiplies four time. you will See that it is a Concave Spindle with a thread passing round it, like a Screw or Circularly inclined plane: the perpendicularity of the thread increases as the Spindle decreases, & the contrary, So that the motion & power are equable, & the motion communicated by the wheels 1. 2. 3. & 4. being press’d down the inclin’d plane, each wheel causing a revolution in the Spindle. It required a great deal of time & attention to proportion So that the wheels might roll down perfectly free. but without a Model, its not easily described6 but is extremely7 Simple.—I hope you will Considder the Subject I have been writing on, & particularly relative to the operation of water on the Spindle before mentioned.

Take a cylinder of wood about 9 inches long & 2 in diameter lay it out with two threads each going half round, Cut away the Supperfluous wood, Stick a cupple of pins in the end, & Cover it with paste-board, then place it in a perpendicular8 possition,9 & pour wa[ter] into it keeping it full, & I think I can venture [to say?] that you will be highly pleas’d; by doing this you can give a more Satisfactory opinion of it, you will be convince’d on trying it that the water has a more powerfull opperation than in any other way.—I have made10 but few experiments as to the operation of Steem on the Circularly inclined plane, but from the experiments I have made & from reason, I believe it is the most perfect Steem engine.—it will give me great pleasure when you peruse this paper if you can tell me that you think my time has not been Spent totally in vain.11—its utility in pushing boats through the water must be very great, the operation will be Silent & constant, & is exactly on the principle of Sculling only more perfect.—

Sir with the greatest respect I am your12 most obt & humble Servt

David K. Hopkins

NB if you will be So condescending as to give this Some considdeeration you will please13 direct the result to the care of Matthew Hopkins Esqr. Snow Hill Maryland—

David K Hopkins

RC (MHi); dateline between signature and postscript; torn at seal; addressed: “Thos Jefferson Esqr Montcello Virginea”; franked and postmarked; endorsed by TJ as received 22 Nov. 1809 and so recorded in SJL.

David K. Hopkins (1781–1856) was a resident of Worcester County, Maryland, with a deep interest in the mechanics of motion. He visited TJ in Washington in 1806 and wrote him last year seeking TJ’s permission to lay his research before him, to which TJ responded encouragingly but noncommittally (Hopkins’s tombstone at cemetery of Makemie Memorial Presbyterian Church, Snow Hill; Hopkins to TJ, 26 Sept. 1808, and TJ to Hopkins, 19 Oct. 1808 [DLC]).

1Manuscript: “obseved.”

2Manuscript: “though.”

3Manuscript: “the the.”

4Manuscript: “a.”

5Manuscript: “commuicate.”

6Manuscript: “desribed.”

7Manuscript: “extemely.”

8Manuscript: “perpendicula.”

9Word interlined in place of “direction.”

10Manuscript: “mde.”

11Reworked from “been totally lost.”

12Manuscript: “you.”

13Manuscript: “pleas.”

Index Entries

  • Hopkins, David K.; and hydraulic screw search
  • Hopkins, David K.; identified search
  • Hopkins, David K.; letters from search
  • Hopkins, Matthew (of Snow Hill, Md.) search
  • Humphreys, David; mentioned search
  • mills; mechanics of search
  • mills; wind search
  • steamboats; and circular inclined plane search
  • steam engine search
  • windmills search