BUYS-BALLOT'S LAW. 113 



cnlarly to Holland, Professor Buys-Ballot determined the Law which is 

 known by his name, and is briefly stated in these words : — 



Buys-Ballot's Law. 



If, on any morning, there be a difference between the barometrical 

 readings at any two stations, in Holland, a wind will blow on that day, 

 in the neighbourhood of places on the line joining those stations, which 

 will be inclined to that line at an angle of 90°, or thereabouts, and will 

 have the station where the reading is lowest on its left-hand side. 



(33.) Dr. Buys-Ballot explained this Law, and the reasons for propound- 

 ing it, and its application to an apparatus, termed an Aeroclinoscope, for 

 indicating, each morning, at each port in Holland, the wind which may be 

 expected to blow there during that day, in a pamphlet, dated I860.* 



This Law is the same as that known to hold good in the case of Tropical 

 Cyclones, and also proved to be true in the case of winds in these latitudes, 

 by Dr. Lloyd, in his " Notes on the Meteorology of Ireland," 1854. How- 

 ever, Professor Buys-Ballot was the first to insist on its importance as a 

 means of foretelling wind, both as to direction and force. 



(34.) The further investigation of this Law, and its application to weather 

 predictions, were vigorously carried on by Capt. Henry To3nibee, F.E.A.S., 

 — a name well known to seamen — in connexion with the Meteorological 

 Office ; and in his Eeport on Isobaric Curves, 1869, he makes the follow- 

 ing remarks on the general subject. 



"Whilst considering Buys-Ballot's Law, I have been struck by a few 

 facts which seem to indicate its relation to the general circulation of the 

 air in moderately high latitudes. Here it may be well to repeat that Law, 

 viz., ' If you turn your back to the wind (however light) in the higher 

 Northern latitudes, there will be a lower pressure to your left than to your 

 right hand.' This order is reversed in the Southern Hemisphere ; hence 

 it is probable that as the Equator is approached the wind will blow 

 directly from a high towards a low pressure. 



" With regard to the winds of the Northern part of the North Atlantic, 

 during our winter a high barometer exists over the cold land in Europe 

 and America, whilst it is relatively low over the sea, hence the prevalence 

 of strong South- Westerly winds on our side of the Atlantic, and of North- 

 westerly winds in Newfoundland, as given in Mr, Buchan's work.f These 

 winds work round an area of low pressure over the water, which seems to 

 account for the prevalence of North-Easterly winds in Greenland. 



" Then, as spring advances, the air over the land becomes heated, 

 causing the barometer to fall, whilst water, changing its temperature much 

 more slowly, does not act so quickly on the air above it, and the result is 



* Eenige Regelen voor aanstaande Weersveranderingen in Nederland voornamelijk 

 in verband met de dagelijksche Telegraphische seinen. Utrecht; 1860. An English 

 version of this is " The Foretelling of the Weather in connexion with Meteorological 

 Observations," by Captain F. H. Klein. Translated by A. Adriani, IM.D., 1863. 



t " A Handy Book for Meteorology," by Alexander Buchan, Second Edition, 1868. 



N. A. 0, 16 



