order to explain it I must enter into a brief account of 

 some simple meteorological principles. 



I suppose that all of my hearers have at least learnt 

 from the daily reports in the newspapers that storms do 

 not depend on the actual height of the barometer at any 

 time at any one station^ but on the difference between 

 the simultaneous readings at adjacent stations, on what 

 is called the gradient, the difference between barometrical 

 readings over a given distance, say fifty miles. 



This will best be understood by reference to a diagram 

 of one of our severe storms. The lines on such a diagram 

 are called isobars, and they represent lines of equal baro- 

 metrical readings. Where such lines are closely packed 

 together there are great differences in barometrical read- 

 ings for the unit of distance, that is, there are high gradients, 

 and consequently strong winds. Where the isobars are far 

 apart there is little difference in readings for the unit of 

 distance, and the winds are light. 



In every case also the wind blows so as to have the 

 lowest barometer on its left hand side, in this hemis- 

 phere, and its force is more or less proportional to the 

 gradients. 



It is on this principle that the Dutch mode of weather 

 signalling is based, and this is essentially not a mode of 

 storm warning. The inventor of the apparatus, called the 

 Aeroklinoscope, is Professor Buys Ballot, of Utrecht, and 

 he has formulated the principle which I have mentioned, 

 into what is known as his Law, which for the northern 

 hemisphere runs thus. 



" Stand with your back to the wind and the barometer 

 will be lower on your left hand than on your right." 



The apparatus consists of a bar or cross piece like a 

 ship's yard, hoisted on a mast, which can be swung round 



