118 MOTIONS AND PEESSUEE OF THE ATMOSPHEEE. 



and several other gentlemen ; but Mr. Thomas Stevenson, C.E., in 1867,* 

 vras the first who proposed to define the dimension or magnitude of these 

 atmospheric waves as "Barometric Gradients," a term since adopted by 

 the Meteorological Office. Its further development, and the application 

 to weather predictions, have been carried out by the superintendents of 

 that office, E. H. Scott, M.A., the Director, and Captain H. Toynbee. 



Mr. Eobert H. Scott, M.A., in his work entitled " Weather Forecasts 

 and Storm Warnings," page 48, says : — " No very precise relation has yet 

 been established between the amount of the gradient and the force of the 

 wind, but a gradient of 0-02 inch per 15 miles indicates the probabiUty of 

 as much wind as an ordinary yachtsman likes to meet with." The 

 Standard Gradients adopted by the Meteorological Office are expressed in 

 hundredths of an inch of the mercury for a distance of 15 nautic miles, a 

 measurement almost identical with that adopted on the Continent, of 

 1 millimetre for 60 miles. 



The system of announcing the probability of a coming storm, inaugurated 

 by the late Admiral FitzEoy, having been abandoned, it may be well to 

 briefly allude to the system now in use, and this will be best done by 

 quoting the words of the Eeport by Eobert H. Scott, Esq.f 



" In order to test the truth of this Law (Buys-Ballot's) as appHed to 

 our own daily Weather Eeports, and its value as indicating the approach 

 and progress of storms, I have investigated these Eeports for a period of 

 nine months, viz., from October to December, 1864, and from October, 

 1867, to March, 1868, inclusive. Captain Toynbee, the Marine Superin- 

 tendent of the Office, has also been engaged in the study of these Eeports. 

 " The observations discussed are all those taken at our own stations 

 within the limits of the British Islands, and, in addition, those from Brest, 

 L'Orient, Eochefort, and the Helder. In 1864, the Helder was not a 

 reporting station, and accordingly Heligoland was taken instead. All these 

 stations, except the last named, are situated within the area of a circle, 

 700 miles in diameter, whose centre is at Birmingham. 

 " The mode of investigation was threefold : — 



"I. To ascertain the greatest difference of pressure existing between any 

 two stations within the area, at 8 a.m. on each day, with the resulting 

 direction for the wind according to the Law, and compare therewith the 

 strong vnnds reported as having occurred during the twenty-four hours 

 which succeeded the time of the observed barometrical difference. 



" II. To extract the daily barometrical differences, and the resulting 

 directions for the wind, along certain lines within the area, and compare 

 therewith the winds recorded in the vicinity of those stations during the 

 succeeding twenty-four hours. 



• In the Report mentioned in the note below.t Mr. Scott says : — " Mr. Stevenson 

 measures his Gradients as an inch of mercury in a variable number of miles. I measure 

 mine as a variable number of hundredths of an inch of mercury per 100 miles. 

 Mr. Stevenson's Paper contains, as far as I am aware, the first proposal to estimate 

 the intensity of storms numerically. It was printed in the ' Journal of the Scottish 

 Meteorological Society ' for January, 1868." 



t "Report of an Enquiry into the Connexion between Strong Winds and Eai-ometrical 

 Difierences," by Robert H. Scott, M.A., F.R.S. 



