214 ON TIDES AND TIDAL ACTION IN HARBORS. 



inequality. It is to be presumed in tbis instance that the tides reaching 

 Galveston through the straits of Florida and through the passage be- 

 tween Cuba and Yucatan differ by six hours in their periods, causing 

 the low water of one to coincide with the high water of the other, thus 

 sensibly destroying the semi-diurnal tides, except in so far as they are 

 unequal. This leaves a small tide outstanding, having substantially 

 the form of the diurnal inequality, and producing the appearance of the 

 " single-day tide," or one high and one low water in every twenty-four 

 hours. This residual fluctuation is well marked at times when the 

 moon's declination is considerable on either side of the equator, but dis- 

 appears almost entirely when the moon is near the equator, since, at 

 such times, the diurnal inequality disappears. Tides of tbis class have 

 always a small range ; in tbe Gulf of Mexico they rarely exceed two and 

 a half feet, and the average rise and fall is but one and a half foot. 



Tbe tides on the coasts of the United States have been specially in- 

 vestigated by Professor Bache, tbe late Superintendent of tbe American 

 Coast Survey. In connection with that work he organized an extensive 

 system of exact tidal observations, for the purpose of ascertaining the 

 complicated laws which govern the tides of the seas that wash our 

 shores. It will be readily understood that in order to separate the 

 effects of the different causes which modify the phenomena, it is not 

 sufficient to observe merely the heights and times of high and low 

 water, but that a continuous record of the tides is necessary, as the 

 inequalities are constantly shifting their place and magnitude. 



TIDE-GAUGES. 



For tbis purpose a self-registering tide-gauge is used, by which a 

 continuous curve, representing the successive changes in the height of 

 water, is traced on paper, moved by clock-work, by a pencil actuated by 

 the rising and falling of a float in a vertical box, to which tbe tide has 

 free access. The time-scale Is such that every hour is represented by 

 one inch, and is pricked into tbe paper by points on the cylinder which 

 moves tbe i)aper forward. The scale of heights is so adapted to the 

 range of the tide at the place of observation that the extreme range of 

 the curve will not exceed tbe width of tbe sheet — twelve inches. A con- 

 tinuous sheet, sufficient for the record of a whole month, is put on tbe 

 tide-gauge at one time. A complete description of this instrument will 

 be found in the United States Coast Survey Report for 1853. [The 

 lecturer illustrated tbe construction of several tide-gauges by means of 

 diagrams.] 



In northern ports interruptions are experienced in winter from the 

 float-box becoming clogged with ice, and various devices have been 

 resorted to for overcoming tbis difficulty. One of tbe most effective has 

 been that of maintaining a temperature above freezing within tbe float- 

 box by means of a simple heating-apparatus. An arrangement of this 

 kind has actually been used on the Fox Islands, in Penobscot Bay. A 



