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rily glanced at on the weather-map; but a moment's reflection shows the importance of accuracy. A fall of only one tenth of an inch in the barometer, trifling as it appears to be, indicates the presence of an enormous force. The normal atmospheric pressure, which is only 15 pounds upon a square inch, is 2,160 pounds upon a square foot, and this amounts to about 80,000,000 tons upon every square mile. With this pressure the column of mercury in the barometric tube is 30.00 inches high. When, therefore, it falls to 29.90 inches only, as any one may calculate, over 100,000 tons of pressure are lifted up and removed from every square mile over which the diminution of pressure extends. Conversely, if the barometer registers on its scale so sinall an increase of pressure as one tenth of an inch, it indicates the arrival of a new mechanical power exerting an additional force of 100,000 tons weight to the square mile. Such minute but common barometric changes, representing forces of great moment in the operation of the atmospheric machinery, must not be overlooked in the deductions of practical meteorology. But without the weather-map of simultaneous observations the presence and influence of such changes can not be detected and estimated.

Signal Service Instruments.-The necessity for accurate observations in a system of weather-telegraphy brings us to speak of the instruments employed by the Signal Service Corps. These have been selected from the best models known, and subjected to experimental tests to perfect their registrations. Every barometer, thermometer, or other instrument used at the stations undergoes thorough comparison with the highest standards before it is sent out from the office of the Chief Signal Officer, in which there is a large apartment devoted to the work of instrumental ineteorology, known as instrument and model room."

"the

The barometer is the great dependence of the meteorologist, and upon its faithful accuracy in registering the subtile yet momentous changes of atmospheric pressure he must chiefly rely. It measures the weight, at the spot where it is located, of a column of air having a diameter equal to that of its own tube. It has been in use since the beginning of the eighteenth century; but not until 1853, when Adie constructed a marine barometer, did it accomplish its work satisfactorily. Fortin's barometer, however, became the most satisfactory for stations, since it has the best cistern (having a flexible base with a glass cylinder), gets rid of some

uniform a temperature as practicable, and in a vertically suspended wooden box which can be closed when the observer is not taking observations. For purposes of comparison and the detection of any error, as well as to have a substitute in case of accident, two barometers are supplied to each station. Each instrument after it comes from the maker's hands is subjected to the Signal Office tests, and the residnal errors are determined by comparison with the great standard barometer kept at the office, when a certificate of corrections is made out and attached to the instrument; it now becomes a standard itself. Its readings may deviate to a very slight extent from those of the "regulator"; but such deviations being known to a thousandth part of an inch, allowance is made for them whenever the observer makes his barometric report. As the elevation of the barometer above sea-level is determined for each station, the proper correction for that is also applied at each reading.

Great care is taken in the location, correction, and reading of the Service thermometers, which are of the highest standard. The instrument is placed in the open air, so situated that it will be always in the shade and yet have a free circulation of air around, but exposed to no currents of air, and beyond the influence of

any artificial heat. Its surface is also carefully protected and freed from rain or moisture of any kind, and its bulb so placed as to have no contact with the metallic scale or back. Every thermometer sent out to a signal-station undergoes several previous crucial tests, and is brought up to the standard kept in the instrument-room of the central office, where every error is corrected and recorded, and the character of the instrument fully studied. The maximum and minimum thermometers are likewise tested, and the slightest variations from the standard instruments determined by protracted experimentation, to the satisfaction of the office, before they are issued to the obThese instruments, by constant and minute inspection of an officer detailed to visit all the stations, as also by the rigid scrutiny of the observers themselves, are kept up to the highest point of accuracy and precision. In the instrument-room of the Washington office, 1,105 meteorological instruments were last year carefully compared with the "official standards," and 982 were issued to the stations.

servers.

The rain-gauge employed is also constructed with the utmost precision, to register the amount of precipitation to inches and tenths of an inch. This instrument is placed with the top at least twelve inches from the ground, and where it will not be affected by local peculiarities or obstructions from any object near by, so that the rain as it descends from the clouds may be fully caught and measured. It is

fixed firmly in a vertical position, and beyond the risk of being tampered with by unauthorized hands. The rain-water collecting in it is measured by a measuring-rod, graduated to inches and tenths of inches; snow is melted and then measured in the same way.

The wind-velocity measurer or anemometer, which up to the present time has been found the most satisfactory, is that of Robinson. It consists of four hemispherical cups revolving in a horizontal plane and communicating their motion to a vertical shaft or axis. In whatever direction the wind blows, these cups will al ways be driven round with their convex sides foremost, since the air presses with more effect into the cups than on their exteriors. Experiments have shown that the velocity of the cups in all cases equals one third of that with which the wind blows, no matter from what point of the compass it comes; and that this relation between the velocity of the cups and that of the winds is independent of the size of the instrument. By an arrangement of beveled wheels every revolution of the cups is made, through the shaft, to revolve a horizontal cylinder carrying a pencil, which marks on prepared paper the total number of revolutions made by the cups. As the distance traveled by the cups is three times that traveled by the wind, the velocity of the latter can be easily deduced.

and printing barometer, Wild's barometer, and Gibbon's barograph-have been for years under careful testing by the office, with the view of securing forms adapted to general use on stations, and also to obtain an instrument so fitted with apparatus and electric wires attached that its action at a remote point may be automatically registered on paper in the Washington office. Although much has been done to settle this question, it is yet unsolved, and it awaits further experimentation.

The International Weather Service. This novel and vast extension of the national work done by the United States weather service is perhaps the most remarkable practical result of the development of modern meteorology. PreIvious to the introduction of the system of "simultaneous" weather-reports by General Myer in 1870, no observations were taken in any country that could be strictly called synchronous, suitable for the preparation of synoptic weather-charts, or that could be regarded as strictly inter-comparable; but, in each country where weather-reports on a large scale were made, they were prepared from daily observations made at moments of time more or less widely separated. Under the old observational methods, concert among the nations in meteorological work was practically out of the question. Not until the new method of simultaneous observations had been put to the test, and a feasible system devised in which all nations could cooperate, was it possible to combine their investigations of the weather into one grand and uniform scheme, for the purpose of observing the aerial envelope of the globe as a unit. To do this required not only a uniformity in the instruments employed, but also a simultaneity in the hours for reading the instruments; that is, that weather-observers all round the globe should take their observations at one and the same fixed moment of physical time. The organization and successful working of a weather bureau upon such a simultaneous system in the United States prepared the way, however, for an international weather service. Accordingly, when in September, 1873, an International Meteorological Congress was convened at Vienna-an assemblage composed of the official heads of the meteorological bureaus of the different powers-an original proposition was made by General Myer, as the Chief

The importance of accurate anemometers was recently illustrated in the storm which overwhelmed the Tay Bridge in Scotland, carrying a passenger-train into the tempest-lashed Firth with instant and total destruction; an accident which, in the judgment of many, would never have occurred had the bridge-constructors and railway authorities possessed anemometric instruments showing the real velocity and force of the gale. In some American storms the wind has been found to blow with the tremendous velocity of from 100 to 138 miles per hour; and it is difficult to find or frame an anemometer which, while delicate enough to register small disturbances, will be strong enough to stand the force of such hurricanes. But the experiments of the Signal Service, it is hoped, will lead to some instrumental improvements in this direction.

But the great question, as respects instruments, with which the Signal Service has been concerned, is to obtain barometers, thermometers, etc., which will be self-recording, and give without manipulation continuous, exact, and graphic registers of the atmospheric fluctuations. Numerous ingenious contrivances of this kind-as Hough's electric meteorograph

Signal Officer of the United States Army, looking toward a world-wide scheme of weatherresearch. General Myer's proposition was to this effect: "That it is desirable, with a view to their exchange, that at least one uniform observation, of such character as to be suited for the preparation of synoptic charts, be taken and recorded daily at as many stations as practicable throughout the world." The author of this proposition had in his report to the United States Congress in 1872 expressed a desire for such a cosmopolitan work-" a grand chain of interchanged international reports, destined with a higher civilization to bind together the signal services of the world"; and the Vienna conference now responded to his overture with alacrity. As well might the United States or Great Britain seek to unravel the mysteries of the Gulf Stream by surveying only that portion of the great ocean-current which impinges on its own shores, leaving unobserved its sources in the equatorial Atlantic and its northeastward deflection from Newfoundland, as to expect to master the mysteries of the atmospheric ocean by studying only the winds and storms which sweep over its own national bounds. The atmosphere is a unit, and to be understood must be studied as a unit. The storms which pass over us all have their "polar" and "equatorial" air-currents; and, to comprehend the forces which conspire to make a single cyclone, we must extend our investigations far beyond our own territorial limits.

The adoption of General Myer's proposition by the Vienna Congress, and the courteous cooperation on the part of all the leading governments of Europe, soon enabled him to collect materials for laying the foundation of the international research. Rapidly expanding in 1874, the exchange of simultaneous reports became numerous enough to admit of making a daily "International Weather Bulletin and Chart"; and on July 1, 1875, the Signal Office at Washington commenced the daily publication of the "International Bulletin," presenting the tabulated results of simultaneous weatherobservations from all the cooperating nations and from the oceans. These reports are in

tended to cover the combined territorial extent of Algiers, Australasia, Austria, Belgium, Central America, China, Denmark, France, Germany, Great Britain, Greece, Greenland, India, Ireland, Italy, Japan, Mexico, Morocco, the Netherlands, Norway, Portugal, Russia, Spain, Sweden, Switzerland, Tunis, Turkey, British North America, the United States, the Azores, Malta, Mauritius, the Sandwich Islands, South Africa, South America, and the West Indies, so far as they have been placed under meteorological surveillance; and also the great ocean-highways, from which the ships of all flags take observations while en route from port to port.

As early as July 1, 1878, in connection with the daily "International Bulletin" issued by the Washington Signal Office, General Myer

began the daily publication of a graphic synoptic "International Weather-Map." This chart covers the whole international network of observations, and is the supplement and key to the daily bulletin, both being based on the same data, and both of the same date. The "International Weather-Map of Simultaneous Observations" (see map opposite) exhibits the aerial phenomena as they actually existed all around the earth at a fixed moment of time; it is, so to speak, a photograph of the atmospheric machinery, picturing its varied movements and delineating its component parts and elements, so as to represent it as a whole-the desideratum of science in all ages. In carrying out this international cooperative enterprise, the Signal Office, by an order of the Secretary of the Navy, receives the daily simultaneous reports from all vessels of the United States Navy, and has the cooperation of the Pacific Mail Steamship Company's vessels, as also that of the White Star Line, the Occidental and Oriental Steamship Company, the North German Lloyd, the American Steamship Company, the Red Star Line, the Allan Line of steamers, and others, whose contributions swell the daily international reports to over five hundred in number. The daily bulletins and charts prepared from the collective data are mailed to every coöperating seaman and civilian observer without charge, as an acknowledgment of his service to science, and constitute in themselves an invaluable meteorological library. In the cases of all maritime observers, the Signal Office bears all expenses for forms, postage, etc.; and when necessary it furnishes the shipmaster with the requisite instruments. The number of observations now made by separate vessels at sea is 122, and all ocean-going ves sels are requested to embark in this system of research. As a striking illustration of the opportunities which a vessel at sea has for aiding in this meteorological work, it may be mentioned that the steamship Faraday, when laying the last Atlantic cable, encountered a severe cyclone in mid-ocean, which, without heaving to, she reported by her telegraphic wire to Europe, noting the successive changes of wind as the different quadrants of the storm passed over her; thus indicating to those on land the direction and progressive velocity of the gale, so that they could calculate the time and locality at which it would strike upon the European coasts. If, as General Myer holds, it is practicable to establish floating stations in midAtlantic, connected by cable with the continent, the reports from such posts would be of incalculable value to British and continental meteorologists in making out their daily weather-forecasts and ordering storm-warnings for their seaports.

The United States is the geographical theatre upon which cyclopean aerial forces of arctic, tropic, and Pacific origin play their mighty parts in the ceaseless conflict, the vicissitudes of which give us the alternations and extremes