Intelligence and Miscellaneous Articles. 455 



tion of the moon's distances from the earth, as evidenced by the va- 

 riation of her horizontal parallax from 54' to 61', is calculated to 

 produce both in the height of the tide and in the lunitidal interval ; 

 as well as the smaller effect attributable to the annual variation of 

 the moon's distances. The highest possible spring tide is that 

 which follows a new or full moon in the month of January, the 

 moon being at the same time in perigee. The first lecture concluded 

 with an explanation of the diurnal inequality, or difference in the 

 heights of the superior and inferior tides depending upon the moon's 

 declination, and the consequent inclination of the axis of the tidal 

 spheroid to the plane of the equator. The moon being in the 

 equator the superior and inferior tides are equal; but when the 

 moon is in north declination the superior tide is greater than the 

 inferior in north latitudes, and less than the inferior in south lati- 

 tudes. The contrary effect is produced when the moon has south 

 declination. 



In the second lecture, the lecturer explained the mode in which 

 tide observations had been conducted, and the manner in which they 

 may be made to exhibit the different inequalities which theory leads 

 us to expect. He particularly instanced the observations made at 

 the London Docks from the year 1808 to the year 1826, from which 

 tables were calculated by Mr. Lubbock, and published in the Philo- 

 sophical Transactions for 1831. Grouping together observations 

 made in different years, but in the same month, and corresponding 

 to the same half hour of the moon's transit, we may eliminate the 

 effects of winds and errors of observation by taking their mean ; and 

 thus a table of lunitidal intervals is constructed for every month of 

 the year, and every half hour of the moon's transit. 



The variation of the intervals in this table is called the calendar 

 month inequality, and this is eliminated by taking the mean of the 

 horizontal columns. The mean intervals are given under the head 

 " mean," in the last vertical column. Two inequalities explained 

 by theory are involved in this table : 1 . the annual inequality de- 

 pending upon the position of the sun in his orbit; and 2. the in- 

 equality depending upon the moon's declination ; for the sun, occu- 

 pying a definite place in his orbit, and the moon following the sun 

 at a given time, she must necessarily have a certain specific declina- 

 tion. Neglecting, therefore, the annual inequality, the same result 

 will be obtained, whether we compute the times of high water by a 

 calendar month table, or by the column of mean transits and a de- 

 clination table. This table, however, does not exhibit the effects of 

 parallax ; it is therefore necessary to group observations according 

 to the values of the moon's horizontal parallax ; and then by sub- 

 tracting these quantities from the corresponding mean values such 

 effects are exhibited. Tables thus constructed show that the luni- 

 tidal interval is diminished, and the height of the tide increased by 

 the increase of the moon's parallax ; a result to which theory also 

 leads us. The greatest variation in the lunitidal interval at the 

 London Docks is about 38"*, the greatest variation in height about 

 IJ feet. The lecturer then proved that the diurnal inequality is 



