BAROMETRIC PRESSURES ON THE GREAT LAKES 



21 



at 8 p.m. minus (6-8) at 8 a.m., at a uniform rate, that no 

 other changes in barometric gradients occurred in the two days, and that 

 the lag of the change in the water surface behind the barometric change is 

 4 hours. The increase in the elevation of the water surface at the gage 

 station will be at a uniform rate from noon to midnight. The total rise 

 will be b m C w . The rise is properly shown in the line on plate 3 marked " BI, 

 4 h Lag. " Counting from the dotted zero line indicated on the drawing, the 

 elevation of the water will be zero during the 12 hours from midnight to 

 noon of the preceding day and will vary from zero to b m C w during the 12 

 hours from noon to midnight on the preceding day, with a mean elevation of 

 0.5 b wl C w during that 12 hours. The elevation of the water surface will 

 remain b wl C w throughout all of the 24 hours of the current day. With 

 reference to the dotted line, the mean elevation of the water surface on the 

 preceding day will therefore be 



24 24 



and on the current day will be b wl C w . Hence, the increase in mean elevation 

 of the current day over the preceding day will be 



Al 



24 



in which 



24 



-12c. 



(31) 



(32) 



TABLE No. 1. 



