381 



The accompanying profile, marked Fig. 2, shows the character of 

 the mean semi-dinrnal tidal wave projected from the modified gene- 

 ral result given in the foregoing Table 2. It gives, for its altitude at 

 its summit, one hundred and fifty-three thousandths (0.153) of a foot, 

 equal to lyVo '^^^^ ) ^'^^ thirty (30) minutes after the time of the 

 moon's meridian transit is still indicated as the average time of lunar 

 high water. We would adopt this mean result in preference to that 

 shown in Table 1, and in the drawing marked Fig. 1. 



From one day before to two days after the period of the moon's 

 conjunction, and opposition to the sun, the observations upon the 

 tide-gauge were made, continuously, both day and night, at regular 

 intervals of fifteen (15) minutes of time apart. This was for the 

 purpose of ascertaining, as near as possible, the time of lunar high 

 water at the period of the spring tides, and also the elevation of the 

 tidal wave at its summit, when influenced by the combined attrac- 

 tion of the sun and moon, acting in the same or nearly in the same 

 direction. 



For this object a separate tabulation was made of all the quarter- 

 hourly co-ordinates which occurred from about twelve (12) hours 

 before, to twenty-four (24) hours after the period of each conjunc- 

 tion and opposition of the sun and moon, from the new moon of 

 January 4th to new moon of June 1st, inclusive.* In this way we 

 hoped to obtain, at each conjunction and opposition, three semi- 

 diurnal tides, each of which would sufficiently approximate in cha- 

 racter to a semi-diurnal spring tide, and a mean of all would tend to 

 eliminate errors arising from the disturbing forces, caused by irregu- 

 larities in the strength and courses of the winds. 



We were fortunate enough to obtain good quarter-hourly observa- 

 tions, for as many as twenty-four (24) of these spring tides, as 

 follows, viz. : 



At the conjunction of January 4th, 

 " opposition of January 18th, 

 ^' conjunction of February 2d, 

 " opposition of February 17th, 

 *' conjunction of March 4th, . . 

 " opposition of March 18th, . . 



* The winds -were so boisterous, and caused so great perturbations of the lake 

 surface, at the periods of the opposition of .June 15th, and the conjunction of 

 June 30th, that we were obliged to reject the observations made at those periods, 

 in making up the co-ordinates of altitude for the spring tides. — J. D. G. 



