How it Works Part 28

AGRICULTURAL MACHINES.

THE THREs.h.i.+NG-MACHINE.

Bread would not be so cheap as it is were the flail still the only means of separating the grain from the straw. What the cream separator has done for the dairy industry (p. 384), the thres.h.i.+ng-machine has done for agriculture. A page or two ought therefore to be spared for this useful invention.

[Ill.u.s.tration: FIG. 226.--Section of a thres.h.i.+ng machine.]

In Fig. 226 a very complete fore-and-aft section of the machine is given. After the bands of the sheaves have been cut, the latter are fed into the mouth of the _drum_ A by the feeder, who stands in the feeding-box on the top of the machine. The drum revolves at a very high velocity, and is fitted with fluted beaters which act against a steel concave, or breastwork, B, the grain being threshed out of the straw in pa.s.sing between the two. The breastwork is provided with open wires, through which most of the threshed grain, cavings (short straws), and chaff pa.s.ses on to a sloping board. The straw is flung forward on to the shakers C, which gradually move the straw towards the open end and throw it off. Any grain, etc., that has escaped the drum falls through the shakers on to D, and works backwards to the _caving riddles_, or moving sieves, E. The _main blower_, by means of a revolving fan, N, sends air along the channel X upwards through these riddles, blowing the short straws away to the left. The grain, husks, and dust fall through E on to G, over the end of which they fall on to the _chaff riddle_, H. A second column of air from the blower drives the chaff away. The heavy grain, seeds, dust, etc., fall on to I, J, and K in turn, and are shaken until only the grain remains to pa.s.s along L to the elevator bottom, M. An endless band with cups attached to it scoops up the grain, carries it aloft, and shoots it into hopper P. It then goes through the shakers Q, R, is dusted by the _back end blower_, S, and slides down T into the open end of the rotary screen-drum U, which is mounted on the slope, so that as it turns the grain travels gradually along it. The first half of the screen has wires set closely together. All the small grain that falls through this, called "thirds," pa.s.ses into a hopper, and is collected in a sack attached to the hopper mouth. The "seconds" fall through the second half of the drum, more widely s.p.a.ced, into their sack; and the "firsts" fall out of the end and through a third spout.

MOWING-MACHINES.

[Ill.u.s.tration: FIG. 227.]

The ordinary _lawn--mower_ employs a revolving reel, built up of spirally-arranged knives, the edges of which pa.s.s very close to a sharp plate projecting from the frame of the mower. Each blade, as it turns, works along the plate, giving a shearing cut to any gra.s.s that may be caught between the two cutting edges. The action is that of a pair of scissors (Fig. 227), one blade representing the fixed, the other the moving knife. If you place a cylinder of wood in the scissors it will be driven forward by the closing of the blades, and be marked by them as it pa.s.ses along the edges. The same thing happens with gra.s.s, which is so soft that it is cut right through.

HAY-CUTTER.

The _hay-cutter_ is another adaptation of the same principle. A cutter-bar is pulled rapidly backwards and forwards in a frame which runs a few inches above the ground by a crank driven by the wheels through gearing. To the front edge of the bar are attached by one side a number of triangular knives. The frame carries an equal number of spikes pointing forward horizontally. Through slots in these the cutter-bar works, and its knives give a drawing cut to gra.s.s caught between them and the sides of the spikes.

SOME NATURAL PHENOMENA.

WHY SUN-HEAT VARIES IN INTENSITY.

The more squarely parallel heat-rays strike a surface the greater will be the number that can affect that surface. This is evident from Figs.

228, 229, where A B is an equal distance in both cases. The nearer the sun is to the horizon, the more obliquely do its rays strike the earth.

Hence midday is necessarily warmer than the evening, and the tropics, where the sun stands overhead, are hotter than the temperate zones, where, even in summer at midday, the rays fall more or less on the slant.

[Ill.u.s.tration: FIG. 228.]

[Ill.u.s.tration: FIG. 229.]

The atmospheric envelope which encompa.s.ses the earth tends to increase the effect of obliquity, since a slanting ray has to travel further through it and is robbed of more heat than a vertical ray.

THE TIDES.

All bodies have an attraction for one another. The earth attracts the moon, and the moon attracts the earth. Now, though the effect of this attraction is not visible as regards the solid part of the globe, it is strongly manifested by the water which covers a large portion of the earth's surface. The moon attracts the water most powerfully at two points, that nearest to it and that furthest away from it; as shown on an exaggerated scale in Fig. 230. Since the earth and the water revolve as one ma.s.s daily on their axis, every point on the circ.u.mference would be daily nearest to and furthest from the moon at regular intervals, and wherever there is ocean there would be two tides in that period, were the moon stationary as regards the earth. (It should be clearly understood that the tides are not great currents, but mere thickenings of the watery envelope. The inrush of the tide is due to the temporary rise of level.)

[Ill.u.s.tration: FIG. 230.]

[Ill.u.s.tration: FIG. 231.]

WHY HIGH TIDE VARIES DAILY.

The moon travels round the earth once in twenty-eight days. In Fig. 231 the point _a_ is nearest the moon at, say, twelve noon. At the end of twenty-four hours it will have arrived at the same position by the compa.s.s, but yet not be nearest to the moon, which has in that period moved on 1/28th of a revolution round the earth.[43] Consequently high tide will not occur till _a_ has reached position _b_ and overtaken the moon, as it were, which takes about an hour on the average. This explains why high tide occurs at intervals of more than twelve hours.

[Ill.u.s.tration: FIG. 232.--Relative positions of sun, moon, and earth at "spring" tides.]

[Ill.u.s.tration: FIG. 233.--Relative positions of sun, moon, and earth at "neap" tides.]

NEAP TIDES AND SPRING TIDES.

The sun, as well as the moon, attracts the ocean, but with less power, owing to its being so much further away. At certain periods of the month, sun, earth, and moon are all in line. Sun and moon then pull together, and we get the highest, or _spring_ tides (Fig. 232). When sun and moon pull at right angles to one another--namely, at the first and third quarters--the excrescence caused by the moon is flattened (Fig.

233), and we get the lowest, or _neap_ tides.

[39] In both Figs. 207 and 208 the degree of expansion is very greatly exaggerated.

[40] As the sun pa.s.ses the meridian (twelve o'clock, noon) the chronometer's reading is taken, and the longitude, or distance east or west of Greenwich, is reckoned by the difference in time between local noon and that of the chronometer.

[41] For much of the information given here about clocks and watches the author is indebted to "The History of Watches," by Mr. J.F. Kendal.

[42] We shall here notice only those gears which are included in the hub of the driving-wheel.

[43] The original position of the moon is indicated by the dotted circle.