52. 53.
Stutter’s Self-recording Rain Gauge. Scale about 1/7.
The funnel has a receiving surface of 100 square inches, protected by a lip 1-1/4 inch deep, to retain the splashes. The rain flows into a copper receiving vessel on the right, which, floating in a cistern of mercury, sinks and draws down with it a pencil, which records the event on a white porcelain cylinder moved by a clock. When the receiving vessel is full the syphon comes into action, rapidly drawing off the whole of the water, the vessel rising almost at a bound, the action being recorded by a vertical line on the porcelain cylinder. Two or more cylinders are supplied with each instrument; and, as the pencil marks are readily removed by a little soap and water, a clean one may be always kept at hand for exchange once in every twenty-four hours.
The Rev. E. Stutter’s Self-recording Rain Gauge is ingenious, and for a self-recording instrument is very moderate in price, while it efficiently shows the rainfall for every hour in the twenty-four (Figs. 52, 53).
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.
[13]. Anemos, the wind; metron, measure.
Constant Winds.—The Trade Winds.—The violent contrast between the temperature of the equator and the poles is well known, and from the vast area included within the tropics ascending currents of rarefied air are incessantly rising and being as incessantly replaced by a rush of cold air from the poles to the equator. Were the earth stationary, this interchange would be of the simplest kind; on arriving within the influence of the ascending equatorial current the air from the poles would be carried to the higher regions and turning over would proceed to the poles, and, becoming cold and dense in traversing the higher stratum, would descend and resume its course ad infinitum. The revolution of the earth on its axis changes all this: the first effect is that the air at the equator is borne along with the earth at the rate of seventeen miles a minute from west to east, a rate which diminishes at 60° of latitude to one-half that velocity, until at the poles it is nothing; consequently a slow north wind flowing to the equator is continually passing over places possessing a higher velocity than itself, and not immediately acquiring that velocity, there is according to the law of the composition of forces a compromise effected resulting in a north-east wind. In a similar manner the same process in the southern hemisphere results in a south-east wind. These winds have acquired the name of Trade Winds on account of the facilities afforded to navigation by their constancy. The North Trades occur in the Atlantic between 9° and 30° and in the Pacific between 9° and 26°. The South Trades occur in the Atlantic between lat. 4° N. and 22° S. and in the Pacific between latitude 4° N. and 23-1/2° S. These limits extend northward with the sun from January to June, and southward from July to December.