576 MR ALEX. BUCHAN ON THE MEAN PRESSURE OF THE ATMOSPHERE 



evident that the first requisite, as regards time, is, that the means be deduced 

 from observations made in the same years. In the tropics, where there is great 

 regularity in the mean pressure of the same month from year to year, observa- 

 tions in the same years are of less importance ; but in extra-tropical regions, 

 where the mean pressure of the same month varies considerably frnn year to 

 year, it is of the utmost importance to obtain observations for the same years. 

 This has been the guiding principle in selecting the years from which the means 

 of the different places in Table I. have been calculated. Thus, in the British 

 Islands, the means are uniformly given for the eleven years from 1857 to 1867, 

 and the means of many European stations are given for the same years ; and in 

 the United States of America the means are for the six years from 1854 to 1859. 



It will be observed, that the means for two or more series of years are given 

 for several places, such as Christiania, Upsal, Toronto, Hobart Town, Algiers, 

 &c. At Christiania, the average for 1861-68 is that for most other Norwegian 

 stations, and that for 1857-67 is the average adopted for the British Islands. 

 For Upsala, the average for 1857-67 is given together with the average for 

 1859-66, the average of the other stations in Sweden. By the comparisons 

 which may be instituted between these means, a closer approximation to the 

 course of the isobaric curves over this portion of north-western Europe is ob- 

 tained. Similar comparisons may be made from the data given in Table I. for 

 different regions of the world, and thus the disadvantages, arising from the 

 necessary use of averages of different terms of years, for different places in the 

 same or in neighbouring regions, may, to some extent, be obviated. 



In addition to these two classes of averages, the averages deduced from long- 

 series of years are given for many places, such as London, 89 years ; Turin, 74 

 years; Bologna, 45 years; Brussels, 38 years; Christiania, 31 years; Toronto, 27 

 years ; Stykkisholm, 23 years ; Hobart Town, 28 years, &c. A comparison of these 

 with the other averages will give some indication of the true mean pressure of 

 the atmosphere for different regions of the globe. But for a general survey of 

 the geographical distribution of the mass of the earth's atmosphere through the 

 months of the year, the data, from which the isobaric lines of the charts have 

 been drawn, may be regarded as sufficient. The closer approximations to the 

 true mean pressure of the atmosphere, to be obtained from the accumulated 

 observations of future years, will give the data for more detailed representations 

 of the pressure of the atmosphere over different regions of the earth. If the 

 isobaric curves could be drawn true for every 0025 inch, the disturbing influence 

 of the Mediterranean, Black and Caspian Seas, and American Lakes ; and of the 

 Pyrenees, Alps, Dovrefeld, Himalayas, and other mountain ranges, would be 

 more apparent. 



In every case, where possible, the means in the Table are the arithmetic 

 means of the observations, reduced to -32° Fahr. only,— no corrections being 



