BAROMETRIC PRESSURES ON THE GREAT LAKES 51 



assumed wind direction. Such a view is indicated in figure 1, plate 4. A 

 nodal line passes through the nodal point of the profile and extends across 

 the lake. Under the action of a wind, the nodal line on the surface of the 

 water remains unchanged in elevation, all parts of the lake surface to leeward 

 of the nodal line are raised, and all parts to the windward are lowered. 



POSITIONS OF VARIOUS NODAL LINES. 



The positions of the various nodal lines for various wind directions and 

 for Lakes Erie and Michigan-Huron are shown on plates 2, 5, and 6. Each 

 nodal line appertains to a wind direction at right angles to that line. 



It will be noted that in various cases there is a short portion of a nodal 

 line extending across a bay, which portion is parallel to but not in line with 

 the portion of the nodal line which is in the main lake. In each of these 



cases the value of S which identifies the nodal line was found by exam- 

 inations of the computation to exist in the bay. These are true nodal lines 

 fixed at certain locations in certain bays, due to the fact that the slopes of 

 the water surface in those bays are as shown in formula (56) , and the eleva- 

 tion of the water surface in the mouth of the bay, where it is first cut off 

 from the lake by intervening land, is as indicated by formula (59). 



On Lake Erie (see plate 2) the nodal line for east and west winds, a line 

 which therefore runs north and south, is much nearer to the western end of 

 the lake than to its middle. It is in the longitude of Pelee Island. Note 

 that when the wind becomes northwest or southeast the main portion of the 

 nodal line still remains in this locality, running, of course, in the southwest- 

 northeast direction, but that it has two detached bay portions, one in the 

 bight northeast of Point Pelee and the other in the bight northeast of 

 Point aux Pins. For north winds and south winds the main portion of the 

 nodal line is in a central position across Lake Erie, but a bay portion extends 

 westward from near the end of Point Pelee. For northeast winds and 

 southwest winds the nodal line is again far from the central portion of the 

 lake in the southwest part. The crowding of three of the four nodal lines 

 toward the western end of Lake Erie is due to the relative shallowness of 

 that portion of the lake which is west of Pelee Island. There the depth is 

 less than 6 fathoms, as a rule, whereas in much of the main portion of the 

 lake the depth is from 10 to 14 fathoms. 



On Lake Michigan-Huron (see plates 5 and 6) the main portion of the 

 nodal line for southwest winds and northeast winds lies in Lake Michigan, 

 not far to the westward of the Strait of Mackinac, and has two detached bay 

 portions, one across Saginaw Bay and one near Port Huron. The main 

 portion of the nodal line for west winds or east winds is in the same locality 

 in Lake Michigan, not far to the westward of the Strait of Mackinac, and 

 has a single detached bay portion across Saginaw Bay. The main portion 

 of the nodal line for northwest winds and southeast winds is in Lake Huron, 



