mint*. 91 



owing to the difference of angle at which the 

 sun's rays strike the earth it is more difficult 

 to account for the violent changes that occur 

 several times during the progress of a season, 

 as well as the less violent ones that come every 

 few days. In fact, it rarely happens that the 

 temperature is exactly the same on any two 

 successive days during the year. The diurnal 

 changes are easily accounted for by the rota- 

 tion of the earth on its axis each day. But 

 there is another class of phenomena with 

 which the "weather man" has to struggle 

 when he is making up a forecast of the 

 weather from day to day. 



In order that we may proceed intelligently, 

 let us say a word about the barometer. We 

 speak of high and low barometer, and we make 

 the instrument with graduations marked for 

 all kinds of weather, which really mean but 

 very little. The reading of a single barometer 

 alone will give us but a faint idea of what is 

 really going to happen from day to day. But 

 if we have a series of barometers located at 

 different stations scattered all over the conti- 

 nent and connected at headquarters by tele- 

 graph, so that we can have the readings fnom 

 a whole series of barometers at once, then it 

 becomes a very useful instrument. A barom- 

 eter may read low at one station by the scale, 

 but may be high with reference to some other 

 barometer that reads very low. 



