2 SCIENCE PROGRESS. 



meter and thermometer — at the majority of them there are 

 a full set of meteorological instruments and some in dupli- 

 cate. The observer has to read the instruments at specified 

 hours, and to telegraph the results to the central office. 



At the central office, the observations from each station 

 are (after the application of any necessary corrections) 

 marked upon a map ; the barometer and thermometer in 

 figures, the wind by an arrow flying with the wind, and 

 with barbs proportional to the strength of the wind. These 

 entries are made over the spot upon the map which repre- 

 sents the station whence those data came. When all the 

 reports have arrived, lines are drawn joining places at which 

 the barometric pressure is the same (isobars), and through 

 those of which the temperature is the same (isotherms). A 

 distinct relation is immediately evident between these iso- 

 bars and the arrows representing the direction and force of 

 the wind. Roughly, the arrows are parallel with the iso- 

 bars, and they are more barbed — i.e., the wind is stronger 

 — the closer the isobars are together. 



Generally these isobars are curved, and enclose an area 

 either of low or of high barometric pressure. In the former 

 case there is said to be a cyclone ; in the latter, an anti- 

 cyclone. If the central barometric pressure be very low 

 and the isobars close together, a violent storm is in 

 progress ; if the central pressure be high and the isobars 

 wide apart there is a calm, and, in winter, generally a fog. 



These cyclonic and anti-cyclonic systems move about, 

 chiefly in easterly directions, but they do not move with the 

 speed of the telegraph, and, consequently, if it were pos- 

 sible to know the direction, and the rate, of their movement, 

 it would be possible to state where the centre woulci be at 

 a given time, and from that knowledge to state what weather 

 would prevail at any given spot, because the weather 

 characteristics of each quadrant of a cyclone are known. 

 The difficulty for the forecaster is, to tell in what direction 

 and at what speed the system will move, and whether it will 

 increase or decrease in intensity. This is not easy even in 

 the United States, where, generally, the systems can be 

 watched from their birth in the Rocky Mountains until they 

 pass away on to the Atlantic ; but for the British Isles and 



