70 



EFFECTS OF WINDS AND OF 



Table No. 16 shows grouped together for convenient inspection, (1) the 

 values of C v and C n derived from the final solutions (see table No. 7, page 

 33), (2) the values of R w and R n computed as indicated in formula (17), 

 page 16, for each gage station, and (3) the various values of P w and P n de- 

 rived by substitution of the values of C w , C n , RW, and R n in equation (19). 



TABLE No. 16. 



If the actual barometric effects were those corresponding to continuous 

 equilibrium of the water under the influence of gravity and barometric 

 pressure, and if all assumptions made in deriving formulae (16) and (17) were 

 true, the computed values of P w and P n would be 1.00 at every station 

 within the limits fixed by the errors of observation. The wide range of 

 values of P w and P n in table No. 16, from 2.83 to +21.17, is ample evidence 

 that decided modifications of the barometric effects are produced by friction 

 and inertia, and that there are probably appreciable errors in the assumptions 

 used as a basis for the computations. The wide range also indicates that 

 the modifications and errors are peculiar to each station, not common to 

 them all. 



The extreme value +21.17 for P w at Mackinaw is probably due to two 

 causes. The gage at Mackinaw is very slightly to the westward of the 

 center of gravity of the area of Lake Michigan-Huron (see plate 2) . Hence, 

 the value of R w for that gage is very small, 0.12. The Strait of Mackinac 

 probably acts as a throttle, to a certain extent, between Lake Michigan and 

 Lake Huron. It probably tends to delay the delivery of water back and 

 forth between the two lakes, which must occur under the influence of baro- 

 metric changes if the two lakes are to act as one. Any delay in the delivery 

 of the water through the Strait of Mackinac tends to make the two lakes act 

 temporarily as separate lakes. If the water surface at Mackinaw gage 

 acted as if it were a part of the surface of Lake Huron alone, the L w of such 

 a formula as (17) for that gage should be measured from the center of 

 gravity of Lake Huron alone and would be much greater than the L w which 

 was actually measured from the center of gravity of the combined lakes. 

 In that case, R w would be much larger than 0.12, while P w would be much 

 smaller than +21.17. In other words, if the oscillation of the water of Lake 



