BAROMETRIC PRESSURES ON THE GREAT LAKES 133 



APPLICATION TO DETERMINATION OF TILTING OF THE GREAT 



LAKES REGION. 



From the evidence derived from gages operated over long periods or 

 during widely separated years, at various points of Lake Michigan-Huron, 

 the eminent geologist, G. K. Gilbert, determined that the whole region 

 covered by this lake is slowly tilting to the southwestward, and secured a 

 determination of the rate of tilting. This determination would obviously 

 be strengthened if the corrections for wind effects and for barometric effects 

 at the gages were applied by using the methods and constants made available 

 by this publication. This investigation by Gilbert was published as a part 

 of the Annual Report of the U. S. Geological Survey for 1896-97, part II, 

 pages 595-647, under the title Recent Earth Movement in the Great Lakes 

 Region. 



The rate of tilting as derived was .0042 foot per mile per century an 

 exceedingly small rate of change. The conclusion was derived from apparent 

 changes of relative elevation of the water surface as measured at different 

 gages on Lakes Michigan-Huron, Erie, and Ontario in different years. 

 The amounts of change involved are of the order of 0.1 to 0.2 foot in 

 a period of 20 to 40 years. Evidently, when such small changes are in 

 question there is more chance of securing the necessary accuracy if cor- 

 rections as large as those shown in tables Nos. 19 to 23, pages 80-96 of 

 this publication, for barometric effects and wind effects, are taken into 

 account. Gilbert limited his deductions largely to days on which there was 

 little wind. But an inspection of the computations made in connection 

 with the present investigation shows that the barometric corrections are by 

 no means negligible on the Great Lakes on days of little wind. Gilbert 

 himself saw the desirability of such corrections, as shown by various state- 

 ments in his paper. 



The deductions of Gilbert are probably correct in the main. But a new 

 investigation based on observed elevations of water surface corrected for 

 wind effects and barometric effects would have greater accuracy and is 

 desirable. Such a new investigation might be in part a recomputation 

 from the data which were used by Gilbert. Much new data of a high degree 

 of accuracy, in the form of observations made in the 24 years since Gilbert's 

 paper was written, are also available. 



