Mean Direction of the Wind. 243 



From a series of hourly observations of the wind, we may infer the 

 best method of dedueing the wind's mean direction from observations 

 made at a limited number ot lionrs. For nine months of the year at 

 Wallingford, the direction of the wind at 1 p. im. corresponds very 

 closely with the mean of the 2-I- hours, while for the other tliree months 

 (May, June and July) this direction is not attained until 5 v. m. The 

 other hour of the day when the wind's direction corresj^onds most 

 nearly with the mean of the 24 hours, is about an hour after mid- 

 night. At Toronto, the two hours when the wind's direction corres- 

 ponds most nearly with the mean of the 24 hours, are 9 a. m. and 6 

 P.M. At Philadelphia they are 10 a.m. and 8p.m. These critical 

 hours appear, therefore, to vary considerably with the locality. 



The hours most generally selected for observations of temperature 

 are 7 a. m., 2 and 9 p. m., and the best result which can be deduced from 

 these observations is obtained by adding twice the 9 o'clock observa- 

 tion to the sum of the other two observations, and dividing the result 

 by four. The same rule gives the true mean direction of the wind at 

 Toronto within less than one degree, although the mean diurnal range 

 amounts to 65 degrees. At Philadelphia also the rule gives an equally 

 accurate result. 



At Wallingford this rule is considerably in error, owing to the fact 

 that the critical hours occur much later than at Toronto ; but during 

 the six colder months, the mean of the 7 a. m. and 2 p. m. obsei'vations 

 corresponds very well with the mean of the 24 hours, while during the 

 other six months, the 2 p. m. observation does not differ greatly from 

 the mean of the 24 hours. 



At most observatories where hourly observations of the wind are 

 made, the observations are not reduced with sufficient accuracy to 

 enable us to test the preceding method of deducing the mean direction 

 from a limited number of observations ; but at Oxford, England, the 

 rule above given for Toronto furnishes a very accurate result. 



The record at Wallingford shows several cases in which the vane 

 indicated the same direction uninterrujjtedly for two days or more. 

 The following examples are selected from the first two years of the 

 observations ; because during this period the foi'ce of the wind was 

 recorded, and we are able to distinguish l)etween the period during 

 which the wind blew with considerable force, and that during which 

 the air was nearly or quite calm. Until January, 1859, the records 

 employed only the eight cardinal points; but subsequently sixteen 

 points were employed. 



The following Table shows first, the direction indicated by the vane; 

 second, the date at which this direction began to be recorded; tliird, 



