SIGNAL SERVICE. 



the next twenty-four hours, is also marked on 

 this chart. (</.) A chart of the normal pres- 

 sure^ those which from a long series of ob- 

 servations should be expected and of varia- 

 tions of the actual from the average pressures. 

 The deviations or "departures" of the actual 

 pressures from those which generally prevail 

 :uv marked on the map by appropriate lines; 

 as also, by the lines of u no variation," the 

 districts in which the atmosphere is in a state 

 of eiiuilibriiun are delineated, (e.) A chart 

 of actual variations of pressure transpiring 

 since the last report, showing the fluctuations 

 of the atmosphere during the previous eight 

 hours. (/.) A chart of dew-point variations 

 which have taken place at the stations during 

 the preceding eight and twenty-four hours. 

 (g.) Lastly, a chart of dew-points, vapor-ten- 

 sions, and the actual amounts of humidity in 

 the air at the various points of observation. 

 All these charts, each covering the whole of 

 the country, must be made out, and the mass 

 of data they embody sifted and analyzed, pre- 

 liminary to the preparation of every one of the 

 tri-daily bulletins issued from the central office. 

 This system of mapping, symbolizing, group- 

 ing, and numerically notating the data enables 

 the Signal Officer to picture to his eye the ex- 

 act status of the aerial masses, and to catch the 



" run " of the weather in the separate sections 

 of the vast field of observation over which it 

 is executing its manifold manoeuvres. These 

 charts have all to be draughted in about an 

 hour or an hour and a hall ; but they are in- 

 ter-corrective, each chart serving as a check 

 on the others. 



Armed with this charted material, the offi- 

 cer preparing the predictions proceeds first to 

 make up the "Synopsis," showing the con- 

 ditions of weather now existing, and then to 

 deduce and write the " Indications," show- 

 ing the changes to occur afterward. As soon 

 as this is done, the deductions are telegraphed 

 direct from the office of the Chief Signal Offi- 

 cer to all parts of the country, and are given to 

 the newspapers. The average time elapsing 

 between the simultaneous reading of the in- 

 struments at the 290 separate stations scat- 

 tered over the United States, and the issue of 

 the "Synopsis" and "Indications" based on 

 these readings, has been calculated at one hour 

 and forty minutes. 



Verifications of Predictions. An analysis 

 of the predictions, made for the year ending 

 June 30, 1879, and a comparison with the 

 weather-conditions which actually occurred 

 within the twenty-four hours next ensuing, 

 give the following percentages of verifications: 



PERCENTAGE OF VERIFICATIONS FOR EACH MONTH OF THE YEAR ENDING JUNE 80, 1879. 



These percentages of accuracy refer to pre- 

 dictions of barometric, thermometric, wind- 

 direction, and general weather changes, which 

 are more difficult to make than those relating 

 to future conditions of the weather alone. The 

 percentage of accuracy of the forecasts of the 

 weather alone (including the state of the skies, 

 whether clear, fair, or cloudy, and whether 

 with or without rain) in all of the different dis- 

 tricts has been 90*7. The percentage for the 

 Pacific coast region is 89*8. Ont of a hun- 

 dred preannouncements of the single element 

 of the u weather" for all parts of the country 

 (apart from barometric changes), ninety have 

 been fulfilled by the event. 



fiiver Reports. The important work of 

 overseeing the fluctuations and floods of the 

 great Western rivers, so sensitive to the me- 

 teorological changes occurring in their basins, 



was at an early period of its history under- 

 taken by the Signal Service. The interstate 

 commerce being necessarily much affected by 

 the oscillations of the rivers, timely warnings 

 of their rise and fall, and daily reports of the 

 exact depth of water at numerous points, were 

 enirerly asked for. The observations of this 

 kind were found of so much importance that 

 they have l>een extended over the Western, 

 Southern, and California rivers, and deduction.* 

 made from them, indicating impending clnnire*. 

 arc dnily published in the Washington weather- 

 reports. All measurements at each river-sta- 

 tion are made from the " bench-mark," as 

 known to the river-men of the vicinity, and 

 the depth of water from the bed of the river to 

 this mark is daily gauged and telegraphed to 

 the central office. Knowing from such tele- 

 grams the height of the river at any station. 



