In addition to all these instruments the observers watch the well-known weather signs in the sky. Sunset and sunrise and the various changes in the appearance of the clouds are carefully studied. When a man has spent a year or two of his life in watching all these things, he can make a pretty safe prediction as to the weather for the next twenty-four hours. The Weather Bureau does not profess to foretell the conditions, except in special instances, for more than forty-eight hours. Now I have told you what the local observers at each station watch and record and note in their reports sent to Washington. What you naturally desire now to know is how do the officials at the central office make their deductions as to the probable weather throughout the country. How do they know that a cold wave is advancing eastward, or that a severe storm is travelling up the coast, and that cautionary signals are to be set between Cape Henry and Passamaquoddy, or some other points? One of the principal ways in which the observers can tell the path of a storm is by watching the rainfall ahead of it. They have found that there is a sort of advance guard of rain, behind which is the lowest barometric area; and they regard that part of the country where the barometer is lowest as the centre of the storm. The reports from various stations show the path of the advancing rain, and the weather observers know that a low barometer is likely to follow it. They cannot tell exactly how fast it will advance, for areas of clear weather stand in the way of the storm, and local causes sometimes prevent them from yielding quickly. [Illustration: COURSE OF CIRCULAR STORM SHOWING ITS TWO MOVEMENTS.]