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 part of the observer may destroy the illusion. Such a mirage may be apparent to a man, and not to a child standing beside him.

The desert mirage is disconcerting as well as deceiving. Surveyors occasionally are compelled to suspend work, and locomotive engineers are sometimes deceived concerning the locations of sidings and signals. A cattfeman who unwisely attempted to drive a herd of several thousand cattle across the Colorado Desert lost the entire herd. The cattle, becoming thirsty, grew very nervous. The mirage deceived them and they stampeded to their death. During a battle between British troops and Turks in the arid plains of the Tigris, a desert mirage concealed the Turks so effectually that fighting was temporarily suspended.

From the nature of the case, forecasts of the impairment of visibility due to refraction are out of the question. However, it may be safely assumed that when a light wind is blowing there will be no trouble from this source. In desert regions the whirls, sometimes known as “sand spouts,” indicate the absence of surface winds. They indicate the breaking up of a lid, but their effect on visibility is slight.

Forecasting Conditions of Visibility.—Some of the fundamental conditions of visibility have been discussed in detail. Fogs cannot be forecast with any degree of certainty, but local conditions may indicate their probability, especially in the case of the coast fogs already noted. Along the steamship lanes between American and British ports, time and place are indicated, not by forecast but by probability. City fogs, which are due largely to pollution, cannot be forecast. They are indicated when the humidity is high and the smoke pollution great. They disappear with a slight rise of temperature. It is well to bear in mind that, with temperature close to the dew-point, a fall of a very few degrees may fill the air with a dense fog; a rise in temperature ever so slight may change foggy air to clear air. Impairment of visibility due to smoke pollution cannot be forecast. When due to a lid, a rise of barometric pressure indicates a clearing of the air, which may take place in an hour. In other words, a lid indicates suspended convection.

The best seeing comes with an anticyclone, the forecasting of which is pretty certain to be verified. Other highs indicate