42 



THE VOYAGE OF H.M.S. CHALLENGER. 



neighbouring station 5269 feet higher, and in January it is only 0°7 lower. Hence, if 

 in these months the temperature of Klagenfurt be used in calculating the temperature 

 of the intervening stratum of air from that place to sea level, it would be much too low, 

 and in all probability the sea level pressure for Klagenfurt would be made nearly 0'030 

 inch above what it ought to be. But even if it be supposed that the temperature of 

 the intervening air stratum could be tolerably approximated to, the barometric observa- 

 tions themselves made in such situations are so strictly local, being largely increased 

 during the cold hours of the day and seasons of the year and diminished during the warm 

 times of the day and of the year, that they would only mislead if used in drawing the 

 isobaric lines of the region where they are situated. Hence in this work the barometric 

 means of such stations as Gries and Klagenfurt in Austria, and Cordova in the Argentine 

 Republic, though printed in the table, have not been used in drawing the isobaric lines ; 

 and all care was accordingly given to keep the maps, from which the isobarics were 

 drawn, clear of sea level pressures deduced from observations made in such situations. 

 It is probable that this consideration explains what look like anomalous observations at 

 a number of places, about the local situation of which there is no information. 



Gravity Correction. — The barometric means in Table VI. have not been corrected 

 for gravity. But as the sea level pressures entered on the maps were reduced to gravity 

 at lat. 45°, the isobars on the maps are corrected for gravity. The following are the 

 corrections for gravity at a pressure of 30 "000 inches which have been used : — 



