382 



At the conjunction of April 3d, ... . None; too stormy. 



" opposition of April ITtli, .... 3 tides. 



" conjunction of May 2d, 1 " 



" opposition of May 16th, 1 " 



" conjunction of June 1st, 3 " 



" opposition of June 15th, ..... None; too stormy. 



'' conjunction of June 30th, .... None; too stormy. 



Total, ... 24 



A direct mean of each quarter-hourly co-ordinate of altitude ob- 

 tained from these twenty-four observed spring tides, is shown in the 

 following Table 3, and the mean spring tidal wave, projected there- 

 from, is shown in the accompanying profile, marked Fig. 3. The 

 whole number of observations incorporated in these, is twelve hun- 

 dred (1^00), and each co-ordinate is here derived from a mean of 

 twenty-four (24) observations. 



Thirty minutes after the time of the moon's meridian transit ap- 

 pears, again, as the time of high water at lunar spring tides, and we 

 have two hundred and fifty-four thousandths (.254) of a foot, equal 

 *^ ^Too(J inches. United States measure, as the difference of eleva- 

 tion of the lake surface between high and low water of spring-tides. 



We designate, as the establishment for the port of Chicago, 



H. M. 



i foot, 0.30. 



It is probable that, if the effects of unfavorable winds, and all 

 other disturbing forces which produce irregular oscillations in the 

 elevation of the lake surface, could be fully eliminated, a semi-diur- 

 nal lunar tide would be shown, at the periods of the maximum 

 springs, as great as one-third of a foot, or four inches. 



The time of low water, and the times of duration of the flood and 

 ebb tides, are given approximately. The extreme rise of the lake 

 tide being so little, and hence the time of the turn from ebb to 

 flood, — attended frequently by disturbances of the winds, — being 

 often uncertain within half an hour, it can only be determined with 

 precision by means of numerous observations made at short intervals 

 of time, say three to five minutes apart, from about one hour before 

 to one hour after the turn of the tide from ebb to flood. 



