BAROMETRIC PRESSURES ON THE GREAT LAKES 3 



each minute. The influence of the slopes and differences of elevation visible 

 to the eye as waves are eliminated automatically, and with an exceedingly 

 high degree of accuracy, by the gages. Such slopes are local in character, 

 they extend continuously without change of sign for a few feet only, and at 

 any one spot on the lake surface they persist of one sign for a few seconds 

 only. The mean elevation of the water surface as recorded by a gage is not 

 appreciably affected by these rapid fluctuations of slope and elevation. 



Omit from consideration the rapid and large fluctuations of slope and 

 elevation of short periods referred to in the preceding paragraph. Just as 

 the effects of these visible wind waves are automatically eliminated, by the 

 gages, from the record written by them, so imagine the surface of the lake as 

 a whole to be freed from the visible wind waves, and imagine each part of the 

 surface to be at its mean elevation a mean covering say 10 to 15 minutes. 

 This smoothed-out lake surface on any one of the Great Lakes will not at 

 any time be level except by accident. It will always be disturbed by wind 

 and by barometric pressures. But the slope in that surface will always and 

 everywhere be too small to be visible to the unaided eye. Anywhere on the 

 Great Lakes where the depth of water is as much as 10 feet and under the 

 most extreme conditions of wind and barometric pressure such slopes are 

 probably not greater than 0.011 foot vertical to one mile horizontal. The 

 slopes are usually less than 0.002 foot vertical to one mile horizontal wherever 

 the depth of water is 20 feet or more; they are, however, as a rule continuous 

 with one sign over the whole surface of the lake at any instant, though at 

 times there is one reversal of sign in passing from one end (or side) of a lake 

 to the other. That is, in general, if the slope produced by the wind and 

 barometric pressures combined is downward from Buffalo to the westward at 

 a given instant, it is downward to the westward continuously all the way to 

 the west end of the lake. Occasionally, and for short periods only, the slope 

 of the character referred to may be downward to the westward in a part of 

 Lake Erie and upward to the westward in the remaining part, or vice versa. 

 The slopes of the character referred to in this paragraph persist with one 

 sign for many hours, as a rule, and sometimes for several days, until the wind 

 changes or the barometric conditions change. 



It is these long-continuous, invisible slopes of the water surface, produced 

 by winds and differences of barometric pressures, that cause the recorded 

 mean elevation, for any hour or any day, at a gage, to be different from the 

 actual mean elevation of the whole surface of the lake for that hour or that 

 day. The difference between the mean elevation of the water surface at a 

 gage for any hour or any day and the mean elevation of the whole lake sur- 

 face during that same time is the combined wind effect and barometric 

 effect. This publication deals with the investigation of these effects of winds 

 and of barometric pressures. It shows how the investigation has been made 

 and how corrections may be applied to eliminate the major part of the 

 effects of winds and of barometric pressures, and so to secure much more 

 accurate values than are otherwise obtainable for the mean elevation of 



