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Description of Mr. MacneiWs neio Instrument.
The instrument above described having been purchased by the Prussian government, Mr. Macneill laid before the Commissioners of Her Majesty's Woods and Forests a design for another, on an improved construction, which has been recently completed for them. The essential difference between this instrument and the former, consists in the self-registration of the effect of every pull or exertion that the horse may make in drawing the carriage, which renders it necessary to have an assistant to mark down the indications of the hand of the dynamometer, which was liable to several objections; for instance, it required some practice to be able to observe accurately the exact divisions marked by the hand, and to write them down with clearness and expedition; also it required that the observations should be made at equal intervals of time, as, for example, five or ten seconds, otherwise the mean power deduced from them would be incorrect. Inattention, therefore, on the part of the observer, was likely to produce an error in the result; and in any case it was difficult to observe and write down a series of observations taken at intervals of ten seconds, which should run on for four or five consecutive hours.
The new instrument, like the former, has a dial with an index, which shows the amount of power exerted at every step of the horse, the vibrations of the index being checked by means of a piston working in a cylinder of oil. But for the purpose of registering the pulls, and marking the power exerted by a horse at every part of the road, a long narrow sheet of paper is drawn, by means of cylinders, over a convex metallic surface or tablet, with a velocity proportioned to the speed of the carriage. The length of paper carried over the tablet is about two feet per mile of the road passed over by the wheels of the instrument. While the paper is thus drawn over the tablet, a pencil or metallic point, which is over the middle of the tablet, traverses the paper at right angles to the longitudinal motion. This point moves by the action of the horse, which pulls out the bar that acts against the spring of the dynamo-meter; but the direction of the motion of this bar being parallel to that of the paper, it is changed, by means of a system of wheels, into a direction at right angles to it, and' also the pencil is thus carried across or at right angles to the line of direction of the bar.
The velocity or space passed over by the point is, by means of the same wheels, increased in the ratio of one to two nearly; thus, if the bar of the dynamometer be pulled out one inch, the point will be carried across the paper nearly two inches. Even the slightest pull on the spring is thus magnified and marked on the paper by the point.
The paper is ruled into faint blue parallel lines throughout its whole length; the intervals represent ten, twenty, thirty, forty lbs., &c, up to 600 lbs.; so that if the spring be acted upon by a power — say ten lbs., the point will be carried over the first interval, or to the line marking ten lbs; if equal to twenty or thirty lbs., the point will be carried to the lines marking twenty or thirty lbs. respectively. These lines will not be at equal distances from each other; for, as the power exerted upon the spring increases, the increments of that power will not be proportional to those of the space passed over by the bar and point; if the former be equal, the latter will diminish progressively, according to the form and strength of the spring; and therefore the distance between the lines must diminish in a certain ratio as the power increases. This ratio may be calculated, but, considering that the friction of all the moving parts must be taken into account, the simplest and perhaps most accurate method is to determine by actual experiment the spaces passed over by the point for every successive addition of weight or power applied to the bar of the dynamometer.
Besides registration of every pull exerted by the horse, the instrument also registers the levels and diversities of the road it passes over; this is effected by means of a heavy weight acting as a pendulum, the vibrations of which are checked by a piston working in a cylinder of oil, and forcing the fluid through the orifice of a tube on the outside of the cylinder which can be enlarged or contracted, so as to regulate the motion with great exactness, according to the nature of the surface passed over by the carriage; an even and smooth surface requiring a large passage for the oil, a rough surface, a contracted one. As the pendulum always hangs vertically, a rod connected with it will move backwards or forwards, according as the instrument ascends or descends; this motion of the rod being in the same direction, or parallel to that of the paper, it is changed, in the same manner as that of the pull, into one at right angles to it; and by this means a point is carried transversely across the paper, in precisely the same manner as the point which marks the pull. If the road be horizontal, the line described by the point will coincide with the zero line. If the road ascend, the bar is moved proportionally out, and consequendy the point is carried to a certain distance from the zero or horizontal line, and will mark on the paper another line parallel to the zero line, so long as the acclivity continues the same; if that increases, the distance of the point from the zero line will also increase. Should the road descend, then the point will pass to the opposite side of the zero line, and by its distance from that line will show the declivity. The lines which mark the rates of clivity should be determined by experiment, as in the case of the tractive power; and this should be done every time the instru