Organized signal services existed in armies from very early periods. Polybius (about 200 B. C.) mentions the wonderful skill acquired by the signal corps of his day. In later years semaphores were used with armies, and codes of flag signals became common for fleets. The invention of the electric telegraph greatly developed organizations of this description. Telegraphic corps are now attached to many armies, and field signals are widely used. Messages of any description, and in words or characters of any language, can be sent by signals, by day or night, as far as one man can by telescopes or other means be made visible to another. The apparatus can easily be carried in the hand on horseback or on foot. To transmit any message' by the use of portable signal apparatus, a distance of 10 m. would be now considered easy. Ranges of from 16 to 20 m. are often reached in ordinarily clear weather; and on the western prairies messages have been transmitted 30 m. by flags. In time of war systems of reports are sometimes organized to cover extensive sections of territory.

In some instances communication can be had from stations on elevated points over the heads of an enemy. - The signal service of the United States army is equipped to maintain communication by signals, by telegraph, or by semaphores, between officers or the different portions of an army or armies, or between armies and fleets. In time of peace it transmits intelligence in reference to storms or approaching weather changes by the display of signals of warning, and by reports at the different cities and ports of the United States. Maps showing the weather conditions are exhibited at board of trade rooms, chambers of commerce, and other places of resort. Bulletins of data are also prominently displayed, and are furnished without expense to leading newspapers. Signal stations are established also in connection with life-saving stations, which are connected by telegraph, and, in addition to displaying storm signals and making the regular meteorological reports, are required to make special reports upon tempests at sea, the sea swell, currents, temperatures, etc. They also summon assistance to vessels in distress, either from neighboring life-saving stations or from the nearest port.

Stations for river reports, to give notice of dangerous floods or conditions of the rivers affecting navigation, are established upon the courses of the great interior rivers. The officers and men of the signal service are instructed for the different branches of the service at the signal school of instruction at Fort Whipple, Va., and at the central office in Washington. They are taught the use of meteorological instruments, the modes of observing, and the forms and duties required at stations of observation, and for the display of storm signals. The force is also drilled with arms and in the usual duties of soldiers. The field telegraph trains of the signal service are organized for use with armies, and are managed by soldiers who are drilled to march with, manoeuvre, work, and protect them. The trains carry light or field telegraph lines, which can bo very quickly erected or run out at the rate of two or three miles an hour. They can be put in use for any distance, and as rapidly taken down, repacked, and marched off with the detachment to-be used elsewhere. - For the duties of the observation of storms, and for the display of storm signals, all stations communicate directly with the signal office in Washington over telegraphic circuits arranged with the different telegraph companies, or connecting with the office at fixed hours each day and night.

Each station is supplied with the following instruments: barometer, thermometer, maximum thermometer, minimum thermometer, Robinson's anemometer with electrical attachment and self-registering apparatus, hygrometer, wind vane, rain gauge, and, on stations located on rivers, lakes, or seacoast, thermometers designed for taking the temperature of water at different depths. The readings of these instruments, made three times a day at fixed hours, are reported to the central office in cipher. The stations at which cautionary signals are displayed are equipped with flags and apparatus for exhibiting the cautionary day or night signal. These stations are established (with the exception of those in the principal cities) solely with reference to the importance of their position for meteoric observations. Three graphic charts are prepared at the central office on the receipt of each report, as follows: 1. A chart of barometric pressures, temperatures, and winds, together with the wind velocities at the different stations, and the precipitation occurring; it exhibits the barometric pressures and the temperatures in their relation to districts and to each other by a system of isobaric and isothermal lines, and the wind directions by arrows at the different stations. 2. A chart of the cloud conditions prevailing over the United States, on which the different varieties and amount of clouds visible at the different stations appear by symbols; on this chart is also indicated the weather as reported at each station, the direction and movement of upper and lower clouds, and each morning the minimum temperature of the preceding night, in relation to districts of territory. 3. A chart showing the relative humidities over territorial districts, with the temperature at the several stations; this enables studies to be made for territorial sections, the difficulties attending the study of observations of this character being obviated to a very considerable degree by the intercor-rections of the* stations among themselves, and by the great extent of the regions over which the readings are simultaneously made.

In the study of the charts for the reports, the well known rules and generalizations established by the experience of meteorologists are used. The published office report, based upon each general report of observations, consists of a synopsis of the meteoric conditions existing over the territory of the United States at the time of the report, and a statement of the changes likely to occur within the next 24 hours. For the purposes of convenient study and of condensed description, the territory of the United States is arbitrarily divided into districts. The reports from the stations, extending over territory reaching from the Atlantic to the Pacific, and from the capes of Florida into British America, are not unfrequently concentrated at the central office in the space of 45 minutes. In military lines connecting frontier posts and lines connecting life-saving stations upon the seacoast, the telegraphic duties are performed by the men of the signal service. The reports are those of. readings of the different meteorological instruments made as nearly simultaneously as possible.

The reports, made simultaneously from all the stations and received at the central office thrice daily, at intervals of about eight hours, arc at once entered graphically upon synoptic charts (the weather maps), and from the study of these charts a deduction is had as to probable weather changes within the ensuing 24 hours. This deduction is furnished to the press and is telegraphed to 21 centres of distribution, to be there published and distributed in bulletin form for the use of farmers. The bulletins are displayed at post offices in numerous villages in the agricultural districts. In the case of serious storms noticed as approaching the lakes, or threatening any part of the seacoast, cautionary signals are ordered from the central office to be displayed at the different lake and sea ports and upon the coasts, as a warning to mariners. The fortunate position of the territory of the United States and its great extent enable a service of this kind to be conducted with especial advantage. The movements of the storms over the continent can be traced upon the charts from report to report, and the direction and rate of their progress together with their intensity be noted in time to give warning of their approach.

Floods occurring upon the western rivers can be traced sometimes from the fall of rain within the respective watersheds, and along the courses of the different confluent streams, until culminating in the dangerous flood of the principal river. In nearly the same manner that storms can be traced upon the charts, approaching changes of temperature and rainfall are foreseen, and notice is frequently given in time to prevent injury to agricultural and other interests. In the analyses of the official deductions of the office, or the "probabilities," the percentage of verifications is found to have been as follows: 1872, 76.8 per cent.; 1873, 77.0 per cent,; 1874, 84.4 per cent. The cautionary signal is a red flag with a black centre by day, and a red light by night. This signal indicates a probability of stormy or dangerous weather for the port or place at which it is displayed, or in that vicinity. While storms of limited extent, such as squalls, tornadoes, etc, may spring up suddenly or pass between stations in such a way that their coming or courses cannot be foreseen, extensive and well defined disturbances can as a rule be readily traced in time to forewarn the coasts or districts threatened.

Arrangements have been made with the chiefs of meteorological services in Europe, in accordance with the recommendation of the Vienna conference of meteorologists (1873), providing for the exchange daily of one report taken at the same instant over all the territories of the United States, nearly all Europe, extending through Russian Asia to the Pacific coast, and in the northern portion of Africa. These exchanges are made every 15 days by mail. Besides the daily bulletins and weather maps, the signal office publishes a weekly review of the weather which is furnished to the press, and a monthly review, accompanied with charts showing the isobaric and isothermal lines, the prevailing winds, the tracks of low barometer, and a precipitation chart for the month.