WEATHER-PROGNOSTICS. 649 



call tlie meteorological significance. "What was the relation of the 

 damp to the rain ? Why did the prognostic sometimes fail ? Why- 

 are there many rain-prognostics associated with a tolerably dry air? 

 Why is not all rain preceded by the same set of prognostics ? To all 

 these questions no answer could be given. Prognostics had almost 

 fallen into disrepute ; they were considered no part of science, and had 

 been supposed to be only suitable for rustics and sailoi's. 



So the subject remained till the introduction of synoptic charts. 

 Then it was soon seen that in temperate regions the broad features of 

 ■weather depend on the shape of the isobaric lines, and later on it was 

 shown — the author believes, mainly by himself — that nearly all prog- 

 nostics have a definite place in some shape of isobars, and that all the 

 above questions, formerly insoluble, receive a ready explanation. It 

 has also been demonstrated that prognostics can never be superseded 

 for use on board ship, and that even in the highest developments of 

 weather-forecasting by means of electric telegraph, prognostics often 

 afford most valuable information. But before we attempt to explain 

 how this is done, we must introduce the reader into the elements of 

 synoptic meteorology. 



Synoptic meteorology is that part of the science which deals with 

 the results obtained by constructing synoptic charts. Formerly, all 

 meteorology w'as deduced from the changes which took place in the 

 instrumental readings at any one place during any interval of time, 

 say one day. For instance, a great deal had been discovered as to the 

 connection between a falling or rising barometer and the accompany- 

 ing rain or wind. Synoptic charts, on the contrary, are constructed by 

 taking the readings of any instrument (say the barometer), or any 

 observations on the sky or the weather (say where rain is falling, or 

 cloud or blue sky is seen), at a large number of places at the same 

 moment (say 8 A. m. at Greenwich). A map of the area or district 

 from which the observations have been received is then taken, the 

 barometer-readings are marked down over their respective places, and 

 then lines are drawn through all the stations where the pressure is 

 equal ; for instance, through all the places where the pressure is 29"9 

 inches (760 mm.), and again at convenient intervals, generally of about 

 two tenths of an inch, say 29 "7 inches (755 mm.), 29'5 inches (750 

 mm.), and so on. These lines are called isobaric lines, or more shortly 

 isobars — that is, lines of equal atmospheric weight or pressure. This 

 method of showing the distribution of pressure by isobars is exactly 

 analogous to that of marking out hills and valleys by means of contour- 

 lines of equal altitude. 



Similarly, the places which report rain, cloud, blue sky, etc., are 

 marked with convenient symbols to denote these phenomena. Then 

 arrows are placed over each observing station, with a number of barbs 

 and feathers which roughly indicate the force of the wind. By 

 an international convention, the arrows always fly with the wind ; 



