580 Journal of Agricultural Research vol. xx.No. ? 



in regard to complete determination of B by W, H 3 and T furnish only 

 seven equations. 



Fortunately, data are given in another paper by Briggs and Shantz (j) 

 from which an eighth equation can be derived. In this paper the average 

 value of each of the measured factors is given for each hour of the day. 

 The cycle of changes in wet-bulb depression follows very closely the 

 changes in temperature. In fact, there should be very little, if any, 

 regular hourly cycle of changes in absolute humidity, so that the wet- 

 bulb depression should be wholly determined by the temperature changes 

 except for some influence of wind velocity. 



Let pB-T = t be the path coefficient which measures the relative influence 

 of temperature on wet-bulb depression in the variations from day to 

 day. Let pB-u = h, pB-w = iv > and let a T , a H , a w , and <r B be the standard 

 deviations of the daily differences in the various factors and in wet-bulb 

 depression. Let T' — T", etc., be the actual differences in temperature, 

 etc., at certain times. The difference to be expected in wet-bulb 

 depression, B' — B" , is as follows: 



B'-B" T'-T" W'—W" , H'-H'' 



= 1-\ w-\ h. 



While t, w, and h are assumed to measure the relative influence of tem- 

 perature, wind, and humidity in the variations from day to day, the 

 foregoing formula should apply under any conditions, if t, w, and h were 

 calculated from a system which represented truly causal relations. 



The expression — t is shown in part I to give the change in wet-bulb 



depression (B) directly caused by a unit change in temperature. The 



relative importance of the various factors in determining the variations 



from hour to hour is very different from that from day to day, but the 



change in wet-bulb depression caused by unit changes in temperature, 



wind velocity, or absolute humidity should always be the same so long 



as the relations are substantially linear. 



The greatest difference in temperature within an average day in the 



data was between 5 a. m. and 3 p. m. This is given as 32. 7 F., or 



18.167 C. The difference in wet-bulb depression between these hours 



was 21.8 F., or 12.111 C. The difference in average wind velocity was 



2.5 miles per hour. The standard deviations of the daily variations have 



already been given. 0-1 = 3.48 day degrees C, <r B = 58 hour degrees C. 



integrated for 24 hours. This means 2.4167 degrees C. 0-^ = 2.24 miles 



per hour. We will assume that there is no difference in absolute humidity 



(H' — H" = o). Substituting those values in the formula for wet-bulb 



depression, we get 



12. in 18.167 , 2.50 



— = ^.f\ — ~w 



2.4167 3.48 2.24 



5.0114=5.2204/+ i.ii6iw. 



