the Mean Annual Temperature Variation. 489 



The principal results gained by these authors may be 

 summarized as follows : — 



(1) The temperature depressions are not strictly bound to 

 given dates. Their occurrence varies slightly from year 

 to year, but on the average, in Central Europe, the 11th- 

 15th of May and the 4th-8th of June show a well-pronounced 

 deficiency of temperature. 



(2) In all cases, of given years, the comparison of the 

 records has shown a progressive displacement of the wave of 

 cold. 



(3) A characteristic distribution of atmospheric pressure 

 over Europe is noticeable. On the ocean a high-pressure 

 area, extending an anticyclonic tongue eastward, moves 

 slowly from the centre of action of the Azores northward. 



(4) The resultants of the observed wind directions change 

 radically. 



(5) Considering a long series of observations, groups of 

 years may be distinguished for which the temperature de- 

 pressions of May and June are well pronounced, while in 

 other groups of years they occur only occasionally. 



This fact applies at least to the Paris * and Geneva f 

 observations. 



The marked departure of temperature conditions of given 

 dates, from the steady seasonal advance, may receive three 

 different interpretations which are illustrated by the following 

 diagram (fig. 2). 



Curve " A " expresses the hypothesis that temperature is 

 below the normal for given dates " K," and that " a' " is the 

 continuation of curve " a." This represents the very gene- 

 rally admitted hypothesis. 



Curve " B " represents the anomaly " p " as being due to 

 such more or less sudden change in the distribution of the 

 isotherms that tw b " belongs to a curve identical with u a," 

 but shifted downwards. 



Curve " C " presupposes a radical modification of the annual 

 variation. At " q " the curve "b" is supposed to have a 

 greater amplitude than u a "; at " r ,} the curve " c " is sup- 

 posed to have a smaller amplitude than " b." 



A careful examination of the available temperature curves 



* E. Renou, Annates Bur. Centr. Met. France, vol. i. p. B. 195 

 (1887). 



f R. Gautier et H. Duaime, Arch. Sc. Phys. ser. 4, vol. xv. p. 545 

 (1903). 



Phil. Mag. S. 6. Vol. 33. No. 198. June 1917. 2 M 



