160 RECORDANDRESULTSOF 



wave by a full line constructed over the average level as an axis of abscissae. The 

 combination of the two component waves will show the features of the diurnal 

 inequahty ; thus, the upper of the two annexed diagrams 

 exhibits the position of the semi-diurnal wave on Novem- 

 ber 30, when the inequality in the height of high water 

 is greatest, and when the low waters show no inequality 

 since they are affected alike. On the contrary, the lower 

 figure exhibts the position on December 8, when there 

 is no inequality in the high waters, and the greatest 

 inequality in the height of low water. In the upper 

 case the maximum ordinates or the high waters of the 

 two waves coincide ; in the lower case they are opposed, 

 or the high water of the diurnal wave coincides with the 

 low Avater of the semi-diurnal. As the semi-diurnal wave 

 progresses or gains on the diurnal all possible variations 

 are gone through successively. For the upper diagram 

 the time of the first low water will be earlier or its luni- 

 tidal interval shorter, and the time of the second low 

 water will be later, or its luni-tidal interval will be 

 greater ; the time of the intermediate high water will 

 not be affected. For the lower diagram the time of the 

 first high water will be later, and that of the second 

 earlier ; the interval of occurrence between these high 

 waters will therefore be considerably shortened. The 

 time of the intermediate low water wiU not be afi'ected. 

 The average range of the diurnal tide for the period 

 represented on Plate III is about three feet, and for the semi-diurnal about seven 

 feet, the greatest and least ranges for these waves are four feet and two feet nearly 

 for the first, and ten feet and four feet nearly for the last. The diurnal wave gra- 

 dually increases in size from the time of the moon's zero declination to the time of 

 its maximum declination, as sho^vn on the Plate. 



The epoch of the diurnal wave appears to remain sensibly the same during the 

 twenty days for which it has been brought out ; that is to say, its high water appears 

 to occur at noon, and consequently its low water at midnight ; the variations from 

 these hours are confined within an hour before or after. The Van Eensselaer 

 Harbor tides afforded but a bare glimpse at the diurnal tide which occurred between 

 October 30 and November 22, 1853, there also its high water appeared to hang 

 about the hours two or three after noon, and its low water the same number of 

 hours after midnight ; but as theory points out a different relation than that of solar 

 time, and conseqiKiiily a gradual sloio sMfiing from the solar Jioui-s, and as our series 

 is too short to show its conformity or non-conformity therewith, we are compelled 

 to leave this interesting branch of the discussion. 



Owing to the variation in the epoch of the diurnal wave, its rate of progress from 

 Port Foulke to Van Rensselaer Harbor cannot be made out directly, since the 

 observations were not contemporaneous, although future observations at some 



