With similar bevel-gear and rods the tangent screw is connected to the hand-wheel, 79, by which the observer communicates the fourth or slowest motion in position angle.
An account of his helicoptere or screw model appeared in the Aeronaut for January 1872, but before giving a description of it, it may be well to state very briefly what is known regarding the history of the screw as applied to the air.
There is thus a thrust outwards of the spring upon the hollow cap W (attached outside the box), and a thrust of the rod upon the end of the screw s.
This block gave a more rigid platen, and at the same time ensured a more equal motion to the screw when actuated by the bar-handle.
On which rubs the block CD, which represents the female screw or bush, and moves between the points E and F, sometimes towards E, sometimes towards F, but having as often to measure short distances as long distances from the middle point of this range, and these as often towards E as towards F.
The degree scale was subdivided to to' and a slow-motion screw at the head enabled differences of one minute to be given; a clamping screw and lever were provided (see fig.
There is a collar, clamped by the screw at S, which is so adjusted that the eye-piece is in focus when pushed home, in its adapter, to this collar.
Boulton and Watt's screw press, invented in 1788 and used at the Royal Mint until 1881, was worked by atmospheric pressure applied to a piston.
The lowest of these slides reposes upon a foundation-plate pp, into one end of which the screw s is tapped.
All this is true, but three readings instead of one for each pointing, much more figure-work in computation (especially if corrections have to be applied to the scale readings to reduce them to exact normal screw readings), are factors which involve a far greater expenditure of time than making a few additional turns of a screw in the process of measurement.