400 Royal Institution : — 



than at others around the British Islands, give frequent premoni- 

 tions ; and therefore great differences of pressure (or tension) shown 

 by barometer, of temperature, of dryness, or moisture, and direction 

 of wind should be considered as signs of changes likely to occur soon. 



It will be observed, on any continued comparison of weather- 

 reports, that during the stronger winds a far greater degree of uni- 

 formity and regularity is shown than during the prevalence of mode- 

 rate or light breezes : and this should be remembered. 



When neither of the greater and more extensive atmospheric 

 currents is sweeping across the British Islands (currents of which 

 the causes are remote, and on a large scale), the nature or character 

 of our winds approaches, and is rather like, that of land and sea 

 breezes in low latitudes, especially in summer. 



Either the cooler sea-wind is drawn in over land heated by the 

 summer sun, or cold air from frosty heights, snow-covered lands, or 

 chilly valleys moves towards the sea, which is so uniform in tempera- 

 ture for many weeks together, changing so slowly, and but little in 

 comparison with land during tha year. These light variables may 

 at such times be numerous, simultaneously, around the compass, on 

 the various coasts of the British Islands. 



Frequently it has been asked, " In this country, how much rise or 

 fall of the glasses may foretell remarkable change or a dangerous 

 storm ? " 



To which can now be replied, great changes or storms are usually 

 shown by falls of barometer exceeding an inch, and by differences 

 of temperature exceeding about fifteen degrees. A tenth of an inch 

 an hour is a fall indicating a storm or very heavy rain. The more 

 rapidly such changes occur, the more risk there is of dangerous 

 atmospheric commotion. 



As all barometric instruments often, if not usually, show what 

 may be expected a day or even days in advance, rather than the 

 weather of the present or next few hours, and as wind, or its direc- 

 tion, affects them much more than rain or snow, due allowance 

 should always be made for days as well as for hours to come. 



The general effect of storms is felt unequally in our islands, and 

 less inland than on the coasts. Lord Wrottesley has shown, by the 

 anemometer at his observatory in Staffordshire, that wind is dimi- 

 nished or checked by its passage over land. The mountain ranges 

 of Wales and Scotland, rising two to four thousand feet above the 

 ocean level, must have great power to alter the direction and pro- 

 bably the velocity of wind, independently of alterations caused by 

 changes of temperature. 



"It not unfrequently happens that a series of cyclones follow 

 closely upon each other for several weeks, the preceding members of 

 the series being often overtaken and interfered with by those suc- 

 ceeding. It is, however, important to remark that, amidst all the 

 complexity necessarily occasioned by such combinations, the greater 

 and more violent storms, and particularly that portion of them which 

 is most dengerous and destructive, exhibit almost invariably the 

 simple cyclonic character. It is thus with the ' Law of Storms ' as 



