304 Proceedings of the Royal Society 
since the mean pressure at Reykjavik, Iceland, during January 
1867, was 29*913 inches; during February, 29*359 inches; and 
during March, 30*037 inches, it is evident that a good many years 
are required to represent the mean of the month at Reykjavik. 
Hence, a very subordinate position, if any at all, has been 
given to observations from such places, unless they embraced a 
considerable number of years. The following are a few of the 
places, with the number of years, for which the means are given : — 
Sitka, 15; Algiers, 10; Hobart Town, 25 ; St Louis, Mauritius, 13 ; 
Bogoslovsk, 26 ; Nigni-Tagilsk, 21 ; Barnaul, 19 ; Nertchinsk, 18 ; 
Pekin, 14; Calcutta, 11 ; Tiflis, 14; Baku, 17 ; Alagir, 15 ; Jakobs- 
havn, 10; Reykjavik, 13; Hammerfest, 13; St Petersburg, 19; 
Archangel, 18 ; Zlaloust, 28 ; Lugan, 22 ; Christiania, Cracow, and 
Kursk, 27 ; Brussels, 33; Gland, 26: Gleneva, 25; Ahun, 34; Ve- 
rona, 73 ; Bologna, 45 ; Milan, 25 ; Turin, 74 ; most of the Austrian 
stations, 14 to 18, &c. 
Tables were prepared ruled with columns for the insertion of the 
following information : — 1. The place and country ; 2. The source 
or authority from whence the observations were obtained ; 3. The 
number of years of the means ; 4. These years specified ; 5. The 
hours of observation; 6. The latitude; 7. The longitude; 8. The 
height above the sea in English feet; 9 to 21. The simple arith- 
metic means for each month and for the year. 
For reducing to sea-level a table was prepared from the formula and 
Table XVI. given in G-uyot’s Meteorological and Physical Tables, 
D, p. 89. This table, calculated for each 5° F. of the tempera- 
ture of the air — from 40° to 90° — was used in all cases where the 
height did not exceed 800 feet. For higher situations, the reduc- 
tion was made by means of Dippe’s method, as detailed in Guyot’s 
tables, D, p. 60. With the correction for height was included the 
correction for daily range. 
For many means I have been indebted to the labours and writings 
of Buys Ballot, Carl Jelinek, Dove, Quetelet, James, and Kuppfer, 
and particularly to Secchi’s admirable abstracts, which have appeared 
from time to time in the “ Bulletino Metereologico.” 
The means, so reduced, were then entered on charts of the globe 
drawn on Mercator’s Projection, from which isobarometric lines 
were drawn for every tenth of an English inch in the difference of 
