SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. JO 



II. THE OBSERVATIONAL MATERIAL 



Since the number of the observations in the i° fields of the dif- 

 ferent decades is often somewhat small, we found it desirable to 

 combine the results of the single observations in fields of i° lati- 

 tude 2° longitude which we may call 2° fields. 



For each of these 2° fields we have taken the mean temperature 

 in the two decade groups February 3 to March 4 and March 15 

 to April 13, respectively. As we shall see in Chapter III, the actual 

 differences in the mean temperatures over the whole region for 

 separate decades are not large. But since the number of observations 

 in single decades are often very small, better mean values may be 

 obtained for the two decade groups by taking the simple mean of 

 all the assembled observations in each of them, rather than to treat 

 the separate decades independently. We have therefore always 



Figure i. The 2° fields of the observations (cross-hatched) along the 

 route Channel to New York and the more southerly 10° fields between 10° and 

 40°of west longitude. The circles with the numbers i to 12 give J. Petersen's 

 stations (1° fields), and the cross-hatched 1° fields with the numbers Li to L3 

 give Liepe's stations. 



taken the group mean as the mean of all the observations without 

 reference to their division between different decades. The values 

 of the surface temperature so obtained will be found in plates i 

 to 14, where also is given the number of observations for each field. 

 In many fields the observational material is so scanty that even 

 the values for the decade groups are doubtful. For these fields the 

 calculations of mean temperatures for the single decades are omitted. 

 However, there is a series of 2° fields along the route Channel- 

 New York, where the number of the observations is sufficient for 

 this purpose. These fields are shown by cross hatching in figure i 

 and also plate 15. In them, we have therefore given the mean 

 values for the single decades, as well as the mean values of all 



