e 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 grass 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.

[Illustration: FIG. 228.]

[Illustration: FIG. 229.]

The atmospheric envelope which encompasses 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 mass daily on their axis, every point on the circumference 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.)

[Illustration: FIG. 230.]

[Illustration: 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
compass, but yet not be nearest to the moon, which has in that period
moved on 1/28th of a revolutio