August 2, 1883J 



NATURE 



33i 



wind, rain, &c. From these it is evident that there is a dis- 

 tinct relation existing between the distribution of pressure and 

 the direction and force of the wind, and temperature and weather 

 generally. A glance at a number of the charts shows that there 

 is nearly always present either an area of low pressure called a 

 cyclone, usually having a circular form, and as a rule moving in an 



easterly or north-easterly direction ; or an area of high pressure, 

 called an anticyclone, also nearly circular in form but almost 

 stationary in position. The wind in all cases also blows nearly 

 parallel with the isobars, having the region of lowest pressure 

 on the left hand. This has given rise to the following simple 

 .aw propounded by Dr. Buys Ballot for the northern hemisphere, 



& 2S%< 



Blue Sk 



Windy Cirrus 



Mares Tails 



Fig. 1. — Cyclone Prognostics. 



viz. "Stand with your back to the wind, and the barometer will 

 be lower on your left hand than on your right." In cyclones 

 the wind circulates round the isobars in the opposite way to 

 which the hands of a watch move, but exhibits usually a little 

 indraft ; while in anticyclones the wind circulates round them 



in the same way as the hands of a watch, but exhibits usually 

 a little outward motion. The velocity of the wind in all cases 

 depends mainly upon the closeness of the isobars ; for the closer the 

 isobars the greater is the difference in pressure, and consequently 

 the stronger the wind. ET-«_i 



Thunderstorm 



Refraction 



29 6 



Fig. 2. — Wedge-Shaped Isobar Prognostics. 



Since therefore nearly all our weather is of the cyclonic or anti- 

 cyclonic type, and is entirely dependent upon the form and close- 

 ness of the isobars, it is by the aid of isobaric charts that the 

 authors have attempted to explain a number of popular prog- 

 nostics, and to associate them with certain kinds of weather. 



The method of research actually adopted has been for many 

 years past to take notes of any good observation of any prog- 

 nostic and put them by in a portfolio with the nearest synoptic 

 chart available ; or preferably with the nearest both before and 

 after. When a sufficient number had been collected they were 



