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A strip of cardboard is arranged so that the sun's rays, concentrated to a focus by a large spherical lens, shall trace a charred line on it when they are unobscured by cloud or mist. The instrument is universal, having divided latitude and diurnal circles, and thus can be easily set for any locality and for any day in the year.

The sunshine is best expressed in percentages of the possible sunshine, if for instance the sun is above the horizon for ten hours and the record is but one hour, then the sunshine received is obviously 10 per cent.

(83) Wind Anemometers.

An Anemometer for the purpose of estimating currents of air has been already noticed in speaking of ventilation; such a delicate instrument as there described would not be suitable for registering the force or velocity of the wind. The public observatories use for the most part Robinson's and Osler's anemometers, the one registering the swiftness of the air current, the other the pressure.

Robinson's anemometer consists of four metallic cups screwed on to the ends of two horizontal iron rods crossing each other at right angles and supported on a vertical axis which turns freely. A train of clock work is connected with the revolving axis by means of an endless screw, and the number of revolutions is recorded on the dial.

Osler's anemometer essentially consists of a plate a foot square kept perpendicular to the wind by means of a vane; the board has attached to it a spiral spring to which an index showing the degree of pressure is attached; by a mechanical arrangement this pressure is recorded by a pencil.

The velocity of the wind can be approximately calculated from the pressure and conversely by the following formula:

Pressure V2 x 005 and velocity=200 x P.

=

SECTION IV.

WATER SUPPLY.

CHAPTER XII.

SOURCES OF WATER SUPPLY.

IN a few cases a water supply is obtained by distilling sea water, such as for instance in the small towns in some parts of Peru where rain never falls and where there are no other sources of supply, but leaving this quite exceptional instance, the sources of supply for the main part fall under the three following heads:-I. Rivers; II. Lakes; and III. Wells.

(84) I. Rivers.

The word naturally includes all running water, whether from springs or streams. The ordinary flow of rivers depends upon springs or other underground drainage, it is only in times of flood that surface waters find their way into the channel. In but very few instances is a stream pure enough to be used directly; it generally has to be stored and filtered. In the majority of instances a water supply from a river can be led into a storage reservoir without pumping, that is by gravitation alone, provided the intake is placed far enough up the river. In other instances pumping must be resorted to.

(85) II. Wells.

Wells are either surface, or deep wells or artesian wells.

A surface well may yield water which comes from a considerable depth, but it is generally merely rain and subsoil water mixed with more or less of the soluble matters of the soil dissolved; occasionally surface wells are very pure. In some parts of Devonshire, for example, surface wells yield water containing but two or three grains of mineral and an unweighable quantity of organic matter.

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