20 
A NEW METHOD OF ESTIMATING STREAM-FLOW 
given by the following four equations for the gages specified, the first three on 
Lake Michigan-Huron, and Marquette on Lake Superior. 
For Milwaukee, Ei for any hour = (4 - 5) (- 4.97) + (3 -6) (+8.44), in which 
(4—5) must be taken for 7 hours earlier and (3 — 6) for 4 hours earlier 
than the hour for which the effect is being computed 
For Harbor Beach, E x for any hour =(4 -5) (+6.94) + (3 -6) (-0.44), in 
which (4—5) must be taken for 6 hours earlier and (3 — 6) for 6 hours 
earlier than the hour for which the effect is being computed 
••(10) 
••(11) 
..(12) 
..(13) 
In the above equations, Ei is in units of 0.001 foot, and the quantities (1~3), 
(2 _ 4), (4—5) and (3~6) are expressed in units of 0.01 inch of mercury. 
For Mackinaw, Ei for any hour = (4 -5) (-2.54) + (3 -6) (-4.38), in 
which (4—5) must be taken for 2 hours later and (3—6) for 1 hour 
later than the hour for which the effect is being computed 
For Marquette, E x for any hour=(l-3)(+2.67) + (2-4)(+2.38), in which 
(1—3) must be taken for 3 hours earlier and (2—4) for 1 hour earlier 
than the hour for which the effect is being computed 
Table 8. — Location of barometric -points 
Point 
Lat. 
Long. 
1 
O 
47.5 

92.5 
2 
Same as Port Arthur 
87.5 
3 
47.5 
85 
4 
45 
87.5 
5 
45 
80 
6 
42.5 
85 
In equations (10), (11) and (12), (4— 5) is the difference between the barometric 
pressure at the point marked 4 on Plate 2, as read from the weather forecast maps 
for any hour, and the pressure at point marked 5. Similarly, (3 _ 6) is the baro- 
metric pressure at point 3 for any hour minus the pressure at point 6. In equation 
(13) the differences (1 — 3) and (2— 4), similarly, are the differences in hourly pressure 
at those points around Lake Superior. 
The barometric pressure is recorded on the forecast maps at the Chicago 
office of the U. S. Weather Bureau at only two times per day, namely, 8 a. m. and 
8 p. m., 75th meridian time. In using equations (10) to (13), the barometric pres- 
sure at the particular points 1, 2, 3, 4, 5 and 6 are interpolated from the isobars 
(lines of equal barometric pressure) at the two times named each day. The pressure 
for any hour of the day is then obtained by linear interpolation between the 8 a. m. 
and 8 p. m. pressures. The substitution of the proper differences in the hourly 
barometric pressures so obtained at the points shown into equations (10) to (13), 
taking account of the lag stated in each case, enables one to compute the hourly 
effect, Ei, of the barometric pressure on the elevation of the water surface at the 
four gages named. 
To obtain hourly elevations of the mean surface of the lake from the hourly 
elevations recorded at a gage, one must subtract the barometric effect, E\, com- 
puted for each hour from the gage reading for that hour. 
The theory involved in equations (10) to (13) is adequately set forth in Pub- 
lication No. 317, as is also the method of deriving the constants. Equations 
