THE AMERICAN BOTANIST. 



29 



date of the vernal equinox, March 21, may be considered 

 the beginning of spring for the northern hemisphere. 

 From the present point of view, however, the date, apply- 

 ing to the northern hemisphere as a vv^hole,has no signific- 

 ance in the tropics or the polar regions, neither of which 

 can be said strictly to have a spring. While the event is a 

 general climatic one, it is also local in character, and is 

 modified for any place by its latitude and geographical 

 position, by altitude or proximity to the sea, and by many 

 minor features of topography. Spring arrives earlier in a 

 sheltered valley or on a southern declivity than on a bleak 

 hillside exposed to the north wind. The successive move- 

 ments of cold waves of ever diminishing force also show 

 that the advance of spring is not like the steady progress 

 of the crest of a flood wave, but fluctuates — now advanc- 

 ing, now retreating, always slowly gaining, so that sum- 

 mer becomes dominant almost inpreceptibly. 



Of the manifold changes following the retreat of som- 

 ber winter, which shall be used as the criterion for the ad- 

 vent of spring ? Many events have been proposed for this 

 purpose, as the breaking up of ice in rivers and the reopen- 

 ing of navigation, the movements of animals, the earlier 

 phrases of the growth of vegetation, and those employed 

 by the meteorologist, as the advance of an adopted iso- 

 therm of mean temperature, or others depending on some 

 supposed relation between climate and the other phenom- 

 ena mentioned. It may be interesting to discuss each of 

 these briefly. 



The first named — the breaking up of ice and resump- 

 tion of navigation — will necessarily find only limited ap- 

 plication in northern regions, where the rivers remain fro- 

 zen throughout the winter. Anyone interested in studying 

 the average dates of the reopening of navigation in the 

 northern rivers and the Great Lakes will find ample ma- 

 terial in the Monthly Weather Reviews. At many places 

 the arrivelof the first boat is an event of much importance. 

 The average date for the reopening of the Hudson River at 

 Albany is March 19, from records extending back to 1786. 



