RIVER HOOGLY. 



263 



I come now to general causes. 



The horizontal parallax of the moon invariably affects the tides ; when 

 that is high the tides are high, and vice versa, to such a degree of correct- 

 ness, that allowing for local causes, I could venture to construct a table for 

 a year in advance, that should not vayy two inches, from the actual tides. 



When the parallax is highest, on the second, or third day, after the 

 full or change of the moon, the highest tide will correspond with these 

 days, as that is the natural period of its greatest height ; should the pa- 

 rallax be decreasing, the highest tide will be on the day of the full, or 

 change ; and should the parallax be decreasing, and near to its lowest, and 

 increase again, after the natural period has passed, the highest tide will 

 be on the fourth day, after the full or change, of the moon. 



The difference of effect between the high or low parallax of the moon, up- 

 on the height of the tides, is about two feet, frequently much more ; and as its 

 variation, as to the time, is shown to be four days, this is of importance to all 

 mariners, as enabling them, in cases of danger, to ascertain by their Nauti- 

 cal Ephemeris, the true state of the tides. No longer need they trust to the 

 partial observation and equally partial theory founded thereon of Pilots 

 and seamen, most of whom have a notion that the dark spring tides are 

 always the highest, that the night tides are higher than the day tides, and 

 that the highest tide must always occur on the second or third day after 

 the full, or change, whereas the parallax of the moon will effectually 

 supercede this uncertainty, and either warn a mariner with his bark on 

 a shoal not to wait till the second day, and lose the springs, or save him 

 from despair, because these days may have passed, and induce him to wait 

 with confidence till the fourth day, after the full, or change, for the highest 

 tide, as the case may be. 



The 



