An eight-day clock with its upright spindle revolves a small funnel with a sloping tube, the end of which passes successively over the mouth of the twelve or twenty-four compartments in the rim of the instrument; beneath each compartment is placed a tube, as shown in the sectional figure. All rain received by the outer funnel drips into the smaller revolving funnel, and flows down the sloping tube, the end of which is timed to take an hour in passing over each compartment, so that the rain, for example, which falls between twelve and one o’clock will be found in the tube marked 1. Each tube can contain half an inch of rain, and any overflow falls into a vessel beneath, and can be measured; the tube which has overflown shows the hour. V.—MOTION. Wind is air in motion. The motion of the air is caused by inequality of temperature. The earth becomes warmed by the sun, and radiates the heat thus acquired back upon the air, which, expanding and becoming lighter, ascends to higher regions, while colder and denser currents rush in to occupy the vacated space. Two points are to be noted in connection with this rush of air which we call wind, viz., its _direction_ and _velocity_ or _force_. Both are estimated scientifically by instruments called Anemometers,[13] while mariners and the dwellers on our coasts have a nomenclature of their own by which to indicate variation in the _force_ of the wind, founded on the amount of sail a vessel can carry with safety at the time. In the matter of _direction_ winds are classed as constant, periodical, and variable. [Footnote 13: _Anemos_, the wind; _metron_, measure.]