154 



TABLE III. (continued.) 



Correction for Annual Variation. Wherever, in the middle lati- 

 tudes of the northern hemisphere, observations of the dip have been 

 made with sufficient care, it has been found that, after elimination of 

 the effects of secular change, the north dip is somewhat greater in 

 winter than in summer. In the 3rd volume of the Toronto Observa- 

 tions, pp. cxxii and cxxiii, the following Table is given as the result of 

 fifteen years of careful observation made throughout at the same spot 

 and according to the same method of observation, comprising 1 920 

 independent determinations nearly equally distributed in the different 

 months, and averaging about 128 determinations for each of the 

 twelve months ; by combining the months equidistant from July 

 (or the middle of the year), the influence of secular change is elimi- 

 nated : 



