20 REPORT OF THE SECRETARY. 



The whole is arranged in tables giving the dates of observation and 

 the authorities from which they have been obtained. Although these 

 tables are doubtless very incomplete, they have been accepted for pub- 

 lication as contributions to the subject, 'to be corrected and enlarged 

 by subsequent observations. 



A series of observations accurately made with properly arranged tidal 

 instruments, such as are employed on the coast survey , and continued 

 for a number of years, would be of much interest to science, as well as 

 of value to commerce in the construction of wharves and the selection 

 of harbors. Such a series has been established under the direction of 

 Captain Meade, United States Topographical Engineers, which, with 

 the observations under the direction of Colonel Graham, at Chicago, 

 will furnish, if continued, the data required. We think it not improb- 

 able that, if the series is sufficiently extended, a law of periodicity will 

 be discovered in the recurrence of the long intervals of rise and decline, 

 and that these will have some relation to a periodical variation of the 

 seasons in a series of years. 



The most remarkable phenomena in regard to the fluctuations of the 

 lakes are those of the fitful oscillations in which sometimes a sudden 

 rise occurs of several feet at a particular place in calm weather, and 

 also a series of minor agitations. The simplest hypothesis for the 

 explanation of these phenomena is, that they are produced by the 

 passage of atmospheric waves, such as are caused by thunder-storms, 

 and perhaps in some cases by water-spouts, across distant parts of the 

 lake. It is well established by observation at the Smithsonian Insti- 

 tution, as well as at other places, that rapid oscillations of the barom- 

 eter accompany the passage of thunder-storms across the meridian. 

 The mercury suddenly descends, then rises a little, and again falls, 

 and after this regains its former level as the storm passes off to the 

 east. A thunder-storm, therefore, in crossing the lake, would cause 

 an elevation of water directly under it, which, in subsiding, would give 

 rise to undulations, and these arriving in succession from every point 

 of the path of the storm would produce effects similar to those which 

 have been noted. 



Since the whole lunar tide of the ocean does not exceed five or six 

 feet, the effect of the moon even on such large bodies of water as those 

 of the upper lakes must be very small. Colonel Graham finds the 

 difference between high and low water at spring tides, at Chicago, on 

 Lake Michigan, to be about three inches and a half, and to occur at 

 thirty minutes after the passage of the moon over the meridian. It is 

 probable that the height of the tide on Lake Superior would be greater. 



