120 A NEW METHOD OF ESTIMATING STREAM-FLOW 
The total part of the change in storage in the ground on any day,viz (rain fall 
on land) minus (evaporation from land) minus (run-off ), which reaches the lake during 
a period of 17 days thereafter (and including the day the change takes place), if the 
rainfall is just sufficient to balance the evaporation from land plus the run-off on 
each day of the 16-day period after the current day, is in the order of 1.5 per cent of 
such change. This figure refers to either Lake Michigan-Huron or Superior, and 
would probably apply, locally, to any of the region around the Great Lakes. On 
very rare days of exceptionally heavy general rainfall, particularly if the ground had 
been soaked by previous rains, this figure may be augmented, but the figure given 
probably holds for most of the days (pages 102 to 117). 
The accuracy with which the elevation of the mean surface of the whole of 
Lake Michigan-Huron may be determined by correcting the observed elevations 
at the three gages, Milwaukee, Harbor Beach and Mackinaw for wind and baro- 
metric effects, inflow, outflow, rainfall on the lake, run-off into the lake and evapora- 
tion from the lake surface, has been increased. It appears that the mean elevation 
of the whole lake surface on any day may now be determined with a probable error 
of only ±0.010 foot. This is less than J^-inch; an accuracy hitherto unattainable 
(pages 81 to 82). 
