96 



The electric state of the atmosphere can likewise be photographically 

 recorded. 



It thus appears that we do not want for instruments and those of great 

 accuracy, but if we proceed to investigate the facts that have become known 

 to us by means of instruments we shall find that these are neither very many nor 

 very important. 



PROGRESS MADE IN THE ACCUMULATION OP FACTS OF IMPORTANCE. 



One of the most important steps made in this direction has been the theory 

 of the Trades and Anti-trades in which we see how, through the influence of 

 the sun upon the equatorial regions, we have an upward current tending 

 from the equator to the jjoles, called the Anti-trades, and a lower current, 

 tending from the poles to the equator, called the Trades. 



The theory of the IMousoon, or the prevailing winds in' the Indian ocean, 

 may here be considei-ed. While the system of Trades and Antitrades is due to 

 the equatorial regions, as a whole being hotter than the polar, the Indian 

 Monsoon is due to the fact that during summer the continent of Asia is much 

 hotter than the Indian ocean. The tendency of land to become hotter than 

 water during the day, and colder during the night, gives rise also on a smaller 

 scale of time to the phenomena known as the land and sea breezes. With i-egard 

 to disturbances of the atmosphere or storms, we know that these are caused by 

 ineqxialities between the pressure of the air at different jilaces, and this 

 inequality of pressure is in its tarn caused by the effects of heat or of cold 

 upon the atmospheric pressure, to which we may add the change of pressure 

 produced by sudden condensation of vapour. Of course, the tendency is for a 

 rush of air to take place from an area of high to one of low pressure, but Dr. 

 Buys Ballot has shown that this is generally materially modified by other 

 considerations ; so that, in fact, in our part of the woild at least, if we suppose 

 a district of low pressure to lie to our left, and one of high pressure to our right, 

 and stand facing the two districts, then the wind will blow from our back. 



This law holds not only in cyclones but in storms of different kinds. 



An important observation was made by the late Admiral Fitzroy that 

 Eui'opean storms generally travel from the west, and this observation has been 

 made much use of by the Meteorological department in London who receive 

 telegraphic intelligence from the south and west of Ireland of atmospherical 

 disturbance the moment they show themselves there. A warning is then sent to 

 those ports to which it is imagined the storm may extend Id its progi-eas east- 

 ward. So desirable is it to have early intelligence of a storm that the siiggestion 

 has been made to have a floating station two hundred miles out in the Atlantic ! 



]\Ieteorologists throughout Europe are now much engaged with the study 

 of storms, and Mr. Meldrum, of the Blauritius observatory, has investigated 

 with much success the storms of the Indian ocean. 



Before leaving the subject of wind, I may mention maps of the pre- 

 vailing -winds in different seasons in different parts of the ocean as a desidera- 



