40 RECORD AND REDUCTION OF THE TIDES. 



Reduction of Tides, Van Rensselaer Harbor, 1853-'54. 



Having given the tidal record in a form ready for use, the observations next 

 require to be properly tabulated for the pui-pose of deducing empirically their laws, 

 and for comparison with theory. In the United States Coast Survey two blank 

 forms are in use for this tabulation; they have in their essential part been adopted 

 as suitable for the Van Rensselaer Harbor tides, and were used Avith permission of 

 the Superintendent of the Survey. They are strictly applicable only for such cases 

 where the diurnal inequality is comparatively small, or is at least not approximating 

 to the production of single day tides. In order to show, at a glance, the general 

 character of the tides under discussion, they were plotted a second time, and are 

 given in Plates I, II, and III; the observations having previously been referred to 

 the same mean level. From these diagrams it appears that the diurnal inequality 

 is not of so great an effect as to render the use of the ordinary method of reduction 

 unavailable ; on the other hand, it is sufficiently large to require a special discussion 

 for time and height. The extension of the series of observations over a whole 

 year must be considered as a fortunate circumstance, since the results thereby gain 

 considerably in accuracy over others deduced only from a few disconnected lunations. 



The tidal record would not be complete without the observations for direction 

 and force of the wind, and for atmospheric pressure ; the reader will find these 

 records in ray discussion of the meteorological material of the expedition, in Vol. 

 XI, Smithsonian Contributions to Knowledge, 1859. 



The following pages contain the first tabulation of the preceding record, viz: 

 column 1 contains the date, civil reckoning, adopted for convenience sake. Co- 

 lumn 2 gives the apparent time (civil reckoning) of the moon's superior and inferior 

 transit over the Van Rensselaer meridian, obtained by adding nine minutes to the 

 time of transit at Greenwich, allowing for a difference of longitude of 4'' 43*" W. 

 The mean time was converted into apparent time by applying the equation of time. 

 The time for the lower transit was obtained by taking the mean of the time of the 

 preceding and following upper transit. Columns 3 and 4 contain the apparent 

 time of high and low water, taken from the record; in some cases a graphical 

 method was resorted to, to obtain the instant of these phases with greater precision. 

 The equation of time has been applied to the mean time in which the observations 

 are expressed. Columns 5 and 6 contain the lunitidal interval between the time 

 of high water and low water, and the time of the transit of the moon immediately 

 preceding, though in some cases, owing to the half-monthly inequality, it may be 

 the second preceding, the establishment being about 11 1 hours. This transit of 

 comparison has been called transit F by Mr. Lubbock,' The next columns, 7 and 

 8, give the height of high and low water, extracted from the preceding abstract. 

 The remaining columns contain the moon's parallax and declination at noon. 



* Sec an Elementary Treatise ou the Tides, l)y J. W. Lubbock, Esq., London, 1839. 



