18 EFFECTS OF WINDS AND OF 



account (in part at least) by the proportionality factors P w and P n derived 

 separately from the observations at each gage station. The accentuation or 

 modification here referred to is that peculiar to the particular locality at and 

 near the gage station, and which is likely to exist in addition to the general 

 accentuation or modification of the wave considered as one unit for a whole 

 lake. 



FORM OF OBSERVATION EQUATIONS FOR BAROMETRIC 



EFFECTS. 



It is desirable to express the relation between the mean barometric effects 

 at any point on Lake Erie on two successive days, on the one hand, and the 

 barometric difference (6-8) and (5-7), see pages 15-16, on the other hand. 

 The development of the corresponding expression for Lake Michigan-Huron 

 will be given later. 



It is proposed to write one observation equation for each day. The day 

 to which the equation is credited will be called the current day and the next 

 earlier day will be called the preceding day. Each equation is to express 

 the change in the barometric effect from the preceding to the current day. 



Let it be assumed that between 8 a.m. and 8 p.m. of the preceding day 

 (6-8) increased by an amount b wi = (6-8) at 8 p.m. minus (6-8) at 8 a.m., 

 at a uniform rate, and that no other changes in barometric gradients occurred 

 in the two days. 



From equation (18), on the assumptions stated and assuming that there is 

 no lag, the increase in the elevation of the water surface at the gage station 

 will be at a uniform rate from 8 a.m. to 8 p.m. and the total rise will be 

 b m C w . The variation in elevation of water surface at the gage will be ex- 

 pressed by the line marked "Bl No Lag" on plate 3. Counting from the 

 dotted zero line indicated on the drawing, the elevation of the water will be 

 zero during the 8 hours from midnight to 8 a.m. on the preceding day, and 

 will vary from zero to b wl C w during the 12 hours from 8 a.m. to 8 p.m. with a 

 mean elevation of 0.5 b m C w during that 12 hours. The elevation of the 

 water surface will remain b m C w during the last 4 hours of the preceding day 

 and 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 



2 24 



and on the current day will be b wl C w . 



Hence, the increase in mean elevation for the current day over the mean 

 elevation for the preceding day will be 



b Wl C w - -b wl C w = --b m C w = b m B wl (20) 



24 24 



in which R 14 



t> Wl l' W 



24 



