CHARTS. CHART SHEWING THE POSITION OF THE TELEGRAPHIC REPORTING STATIONS, JANUARY 1876 WEATHER CHARTS, NOVEMBER 29, 1874, 8 A. M. (REDUCED to face Title to face p. 150 WEATHER CHARTS AND STORM WARNINGS. CHAPTER I. THE MATERIALS AVAILABLE FOR WEATHER STUDY. BEFORE proceeding to describe the charts themselves, or to give an account of their utility in aiding us to form a judgment as to probable weather, it will be well to explain the character of the observations and the nature of the service on which they are based, confining ourselves exclusively to the instruments employed and observations taken at our own telegraphic reporting stations (the outfit of which is necessarily far less complete than that of a fully furnished meteorological observatory); omitting technical descriptions of the instruments themselves. The observations taken at these stations refer to Atmospherical Pressure, Temperature, Humidity, (or the Dampness of the air,) Wind, Rain, Weather, and, at seacoast stations, Sea Disturbance. The drawing of weather charts depends on the com parison of various observations taken at the same time, at several different stations. It is therefore indispensable, that all the observations to be compared should be freed, as far as possible, from incidental inaccuracies and discrepancies, by reducing the conditions at each place to common standards of comparison, and this is done by applying the several instrumental and other corrections. Atmospherical pressure is measured by the barometer, and by this is meant the mercurial barometer; for the aneroid, however convenient it may be for ordinary use as a weather glass, cannot be recognised as an independent instrument, its indications depending entirely on the delicacy of the mechanical appliances employed in its construction. The barometer has a thermometer attached to it, in order to show the temperature of the instrument itself, and this must be read whenever an observation of the height of the mercurial column is taken. The readings of the barometer are always said to be 'corrected and reduced to 32° and to sea level,' and it is necessary to explain these expressions. 'Corrected,' means that corrections have been applied, a, for the error of the scale of the instrument, which has been ascertained by sending it for verification to some recognised establishment, such as Kew Observatory, b, for what is called 'capillarity,' which depends on the bore of the tube, and c, for what is called 'capacity,' which depends on the proportion which the bore of the tube bears to the sectional area of the cistern. 'Reduced to 32°, means corrected according to the reading of the attached thermometer. The column of mercury in the barometer tube behaves like almost all other bodies, being lengthened by heat, and shortened by cold. It is therefore obvious that unless two similar barometers be precisely at the same temperature, they cannot possibly read alike, and hence is apparent the absolute necessity of reducing all readings to the same common temperature of melting ice, 32° F., which has been unanimously adopted as the best standard temperature for the purpose. The temperature of the column being therefore indicated by the reading of the attached thermometer, according as that reading is above or below 32° F. we can discover, by means of printed tables, how much the column is too long, or too short, as compared with what it would be at 32°, and consequently what correction is required for the barometer reading. 'Reduced to sea level,' is a phrase which requires rather more explanation than the foregoing. The barometer, as its name implies, measures the weight of the air, and that weight, of course, depends mainly on the quantity of air which is above the instrument pressing upon the cistern, and keeping up the column of mercury in the tube. If, therefore, two similar barometers be placed one directly under the other, say, one in the attic, and the other on the ground floor, it is evident that there will |