580 MR ALEX. BUCHAN OX THE MEAN PRESSURE OF THE ATMOSPHERE 



To ascertain what relation may subsist between mean atmospheric pressure 

 and prevailing winds, Table II. has beea-prepared, which show's the mean number 

 of days in each month winds from N., N.E., E., S.E., S., S.W., W., and N.W. have 

 been observed to blow. In tropical and subtropical regions, a period of one or 

 two years is sufficient to indicate the average direction, or the prevailing winds 

 of the locality, owing to the steadiness with which the wind blows in these 

 regions; but in temperate and polar regions a considerable number of years is 

 indispensable. The direction of the winds has been generally obtained, or 

 calculated, for the same years as the atmospheric pressure ; but where such obser- 

 vations could not be obtained, care was taken to include in the list only those 

 places for which a sufficient number of years was available, so as to give a good 

 average. As regards the stations in British North America, the shortness of the 

 time during which, in each case, the observations were made, is to a great extent 

 compensated for by the number of places at which observations have been made, 

 and the comparative steadiness of the winds in these high latitudes of America. 

 Valuable assistance was obtained from Professor Coffin's elaborate " Treatise on 

 the Winds of the Northern Hemisphere," though many averages given in this work 

 could not be adopted, being based on an insufficient number of years,— a remark 

 which applies extensively to averages of observations of the wind hitherto 

 published. 



In selecting stations, a preference was given to those which are situated in 

 comparatively level localities, with the view of obtaining as close an approxima- 

 tion as possible to the true direction of the wind. To this there are, however, 

 several exceptions, such as the stations in Norway and in Greenland, these places 

 being given to illustrate the effect of mountain ranges in changing the mean 

 direction of the wind. Stations at no great elevation above the sea were selected 

 in different regions, it being evident that winds observed at great elevations are 

 not suited to an inquiry into the movements of the atmosphere in relation to 

 sea-level pressures. 



It will be observed that the time, or the duration of the prevalence of each wind 

 (N., N.E., E., &c), is the only element taken into account in this inquiry. The 

 element of force has, for several reasons, been neglected:— (1.) The force of the 

 wind has been less generally observed than the direction ; and at very many 

 places where the force has been observed, the observations, from the manner in 

 which they have been made, do not give the materials for arriving at absolute 

 results. (2.) It is well known that the velocity of the wind is retarded by the 

 land as it passes across it ; thus, for example, an anemometer on the west coast 

 of the British Isles registers considerably more wind than one erected at an inland 

 or eastern situation. Also, more wind is registered in rising above the surface 

 of the earth. The effect of local situation on the velocity of the different winds 

 is very great. Hence, whilst the amount of these disturbing influences are 



