United States Weather Maps and other sources, 5 



fourth. The average of each successive three numbers in this 

 column was taken and the results are shown in column fifth. 



In order to determine the influence of the winds on the moist- 

 ure of the air I took the observations of the force of vapor (ex- 

 pressed in inches of mercury) and compared each observation 

 with the mean force for that month, and the given hour. The 

 results for the three winter months are shown in column sixth 

 of the preceding table, and the averaged numbers are shown in 

 column seventh. The results for the three summer months, 

 obtained in like manner, are shown in columns eighth and ninth. 



In order to determine the influence of the wind on the pres- 

 sure of the air, I placed each observation of the barometer in 

 the column having the observed wind at the top, and took the 

 averages of the numbers in each column. The result for the 

 three winter months (subtracting 29 inches) are shown in column 

 tenth and the averaged numbers in column eleventh. Similar 

 results for the three summer months are shown in columns 

 twelve and thirteen. 



On comparing the numbers in this table we see that in win- 

 ter at Philadelphia the lowest temperature occurs with a wind 

 from the KW., and in summer with a wind from a point 

 about 15° west of north. 



Now if we suppose a mass of air to be transferred from a 

 higher latitude to a lower, we should expect that its relative 

 temperature would be the lowest when it moved in a direction 

 perpendicular to the isothermal lines. Observing this rule we 

 should conclude that m winter the coldest wind at Philadelphia 

 must come from a quarter about 15° west of north, provided it 

 commenced its motion from anv place within 600 miles from 

 Philadelphia. But if it came from a distance of over 1000 

 miles from Philadelphia then the coldest wind would come 

 fi-ora a point 80° west of north. But the coldest wind actually 

 comes from a point 45° west of north ; that is, at Philadelphia 

 in winter the coldest wind blows from a point 15° more west- 

 erly than the coldest region about Philadelphia. 



In summer, if we extend the comparison to a distance of 

 1000 miles from Philadelphia, we shall find the coldest region 

 to lie in a N.E. direction ; but if we confine ourselves to a ra- 

 dius not exceeding 600 miles, we shall find nearly the same 

 temperature prevailing in all directions between the limits of 

 N.E. and N. 25° W. But the coldest wind is observed to 

 blow from a point N. 15° W., which lies within the limits 

 above determined. On the whole, we conclude that at Phila- 

 delphia the coldest wind comes very nearly from the coldest 

 region within a distance of from 500 to 1000 miles from Phil- 

 adelphia, with a suspicion, however, that the former is a tew de- 

 grees more westerly than the latter. 



