Mean Direction of the Wind. 



239 



The numbers in botli tables represent degrees counted ironi the 

 north ))oint around the circle by tlie west and south. 



The diurnal fluctuation at Philadelphia during the cold months 

 is greater than at Wallingford, but during the Avarni months it is 

 decidedly less. The fluctuations at P]iila(lel[)hia appear o;rt'ater in 

 consequence of the shortness of the period of comparison (one year). 

 In order to discoAaa- what would be the efl:ect of extending the period 

 of comparison, that month was selected, (May), which at Walling- 

 ford, showed the most remarkable diurnal fluctuations. Table V 

 shoAvs the results of the Philadelphia observations for the inontli of 

 May, for a period of four years. 



Taule V. — Jlonrlt/ 3Ieans. Direction of the Wind for 3Iay at 



Philadelphia. 



The mean diurnal fluctuation is here reduced to 28 degrees, while 

 at Wallingford, for the same month, it amounts to ;]04 degrees. The 

 cause of this great fluctuation at Wallingford, must be very difl'erent 

 from that which operates at Philadelphia ; or if the cause be the 

 same, it must operate with far greater intensity. 



The results of the observations at Toronto, exiiibit a strong resem- 

 blance to those at Wallingford. For the six colder months the mean 

 diurnal change is nearly the same, and the curves re])resenting the 

 change of direction are similar, although the corresponding clianges 

 are not simultaneous. At Toronto the Aviiul is most southerly about 

 an hour after noon, Avhile at Wallingford tlie wind is gt«ierally most 

 southerly about 5 p. m. During the six Avarmer months, the diurnal 

 change of direction at Toronto is nearly as great as at Wallinuford ; 

 and if Ave omit the month of May, it is greater than at Walliiitiford. 

 Moreover, the curves representing the changes of direction at the two 

 places, bear some resemblance to each other; although the cliauge of 

 wind from north to south generally occurs four hours earlier at 

 Toronto than at Wallingford. 



A comparison of these facts naturally suggests the idea, tliat the 

 diurnal change in the direction of the wind is mainly due to inequality 



