TIDAL OBSERVATIONS. Ul 



Investigations made by different methods for a few places, give very discordant 

 results; for London, Mr. Liibbock found 7 inches, for Bristol, Mr. Bunt found 13 

 inches, and Sir J C. Ross, in a late number of the Philosophical Transactions (for 

 1854, Part II), deduced from observations at Port Leopold, in latitude 74° N., longi- 

 tude 91° W., nearly the same value as that given for Bristol, stating that the effect 

 is nearly in the inverse ratio of the specific gravity of the two bodies (mercury and 

 water). 



The subject is open to further investigations, and considering that an increase or 

 decrease of atmospheric pressure in any one place must necessarily be accompanied 

 by currents restoring the disturbed equilibrium, the phenomenon would seem more 

 complex than might at first be supposed. 



Effect of the Wind upon the Mean Level of the Sea. — As this effect is of an entirely 

 local character, the result will be of importance only in so far as it affects the local 

 phenomena of the tides ; in refined tidal discussions the effect of the wind must be 

 eliminated, and for predicted tides the possible influence it may exert, specially 

 when for spring or neap tides, may become a matter of grave interest. Looking 

 over the columns of the wind record in Table I it appears that the prevailing wind 

 is either N. E. or S. W. ; there occur some calms and a few entries of variable 

 winds. 



Tabulating, for each series of observations separately, the mean level reading, 

 referred to the same zero by application of the corrections given, for days of N. E. 

 wind, for days of S. W. Avind, and for days of calms (including variables), the fol- 

 lowing resiilts were obtained : — 



Series I. Mean level with N. E. wind 16.0 feet (15 observations), with calms 16.6 feet (10 obser- 

 vations), with S. W. wind 16.8 feet (3 observations). 



Series II, Mean level with N, E. wind 1Y.5 feet (6 observations), with calms 18.0 feet (15 obser- 

 vations), with S. W. wind 11.9 feet (13 observations). 



With consideration of the number of days of observation in each case, the effect 

 of the wind appears very small, with N. E. wind the level is depressed a small 

 fraction of a foot, and with a S. W. wind elevated by the same amount. A north- 

 east wind blowing off the land, and a southwest wind blowing on it, would produce 

 the effect as stated. Two causes operate against a considerable change in the level, 

 first the open strait giving free passage to accumulated waters, to the northward or 

 southward, and secondly, the protection of ic^-fields, preventing the wind from act- 

 ing on the surface of the sea. 



We have seen that the effect upo^n the height of the tides produced either by the 

 regular oscillation of the half-tide level, or by the irregular changes in the atmo- 

 spheric pressure and the action of the winds, is sufiiciently small at Port Foulke to 

 be safely left out of consideration in our subsequent investigations ; the corrections 

 alone will be needed which refer all observations to the same zero of the height 

 scale ; they are for series I : Between November 17th and 28th, +5.6 feet ; between 

 November 30th and December 3d, -|-3.2 feet; between December 5th and lOth, 

 -f-2.5 feet. For series II: Between June 6th and 20th, +1.4 foot; between June 



