Warnings of expected rises or falls in the great rivers are made with equal regularity, telegraphed, bulletined in frames, and also published by the newspapers, at the different river cities. These daily reports give the depths of water at different points in the rivers' courses, and thus make it easy for river shipping to be moored safely in anticipation of low water, when ignorance might lead to the grounding of the boats on sand-bars or mud-banks. The notices of the probable heights which freshets may reach, are followed by preparations upon the "levees" and river-banks, to guard against overflows. The United States Signal Service is a branch of the army. No one is admitted to it who is under twenty-one years of age. Every candidate has to undergo before enlistment an examination, the chief subjects of which are spelling, legible hand-writing, proficiency in arithmetic, and the geography of the United States, physical and political. Successful candidates are regularly enlisted in the army, as non-commissioned officers, and go through a course of very systematic instruction in military signaling and telegraphy. They are assigned afterward to different posts, where they are required to make observations and report the same by wire three times a day, to the commanding officer at Washington. These observations are made by means of the instruments I have described, and include the different appearances in the sky; and at all the stations they are made at the same hour, according to Washington time. The telegraph gives to the Herald of the Weather and his aids the advantage of hearing from all the hundred and forty-odd observers almost at the same time; and when all this information has been gathered up, studied out, and re-arranged, the same swift servant takes all over the country, again almost at one time, the ripe results of the care and watching of more than seven score persons separated by hundreds and even thousands of miles from the central office. I should like to describe the instruments fully, but must content myself with telling you what remarkable things some of them do. The self-registering barometer, for instance, is made to actually photograph a storm; another is made to draw with a pencil, every hour, figures that show the height of the column of mercury and the condition of the atmosphere. Even the vane, or weather-cock, marks down the direction and force of the wind.