258 



Direction and Force of the Wind. 



The curve lines upon Plate IX represent the mean force of the wind 

 for each hour of the day, and each month of the year. The hour of 

 the day is indicated at the top and bottom of the page. The space 

 between the horizontal lines represents a diflerence of one ounce upon 

 the pressui-e plate. Since the zero of pressure is different for each 

 curve line, the absolute value of the horizontal lines could not be in- 

 dicated upon the chart without creating confusion ; but a reference to 

 the average results in Table VIII, will readily indicate what the zero 

 is. These curves exhibit strikingly to the eye the diurnal change in 

 the wind's force. 



The time of maximum pressure varies from 1 to 4 p. m. ; occurring 

 generally at 2 p. m. in winter; 3 p. m. in spring and autumn ; and at 

 4 p. M. in summer. These hours, during the colder part of the year, 

 correspond very closely with the time of maximum temperature, but 

 during the warmer part of the year they occiir from one to two hours 

 later. 



The average time of minimum pressure is 2 a. m., but varies from 

 10 p. M. to 7 A. ii., between which hours the average change of press- 

 ure is quite small. 



The general form of the curves of pressure at Wallingford is similar 

 to that of the curves representing the observations at Girard College, 

 Philadelphia, but the absolute pressure is different. Table IX affords 

 a comparison of the extent of the diurnal change at the two places. 

 Column second shows for each month the pressure for the hour when 

 it was least at Wallingford, and column third the pressure at the horn- 

 when it was greatest Column fourth shows the minimum pressure 

 at Philadelphia, expressed in pounds per square foot, and column 

 fifth shows the same numbers reduced to the standard of Wallingford, 

 viz : ounces of pressure on a surface of 100 square inches. Columns 

 6 and 7 show the maximum pressure at Philadelphia similarly ex- 

 pressed. The results given for Philadelphia are the means of 1\ 

 years of observations. 



Table IX. — Monthly Maxima and Minima of pressure at Walling- 

 ford and Philadelphia. 



