96 ALEXANDER JOHNSON ON TIDAL 



don for 1833 : ""With regard to this alone of all the consequences of the law of universal 

 gravitation, the task of bringing the developed theory into comparison with multiplied 

 and extensive observations is still incomplete ; we might almost say, is still to be begun." 



Progress has been made since the date of Dr. "Whewell's paper, more especially since 

 the question was taken up by the British Association, at the suggestion of Sir William 

 Thomson in ISBT, and a committee was appointed for the purpose of promoting the exten- 

 sion, improvement and harmonic analysis of tidal observations, which committee has been 

 continued from year to year ever since. Very much, however, remains yet to be done, 

 and we in Canada ought to be able to take our own share in the work, whether we regard 

 the scientific or the practical results to be expected. 



Among the scientific results that may be expected from tidal investigation founded 

 on extensive observations are : — 



1. A determination of the mass of the moon ; since the attraction depends on this. 

 Some thirty years ago this mass was estimated at about one seventy-fifth part of the 

 earth's mass. The value at present adopted is about one eighty-first part. 



2. More definite knowledge of the question, which has been so interesting to geolo- 

 gists, concerning the rigidity of the earth. 



3. The effect of tidal friction in the retardation of the earth's rotation. 



4. Possibly also by the aid of Sir William Thomson's corrected equilibrium theory, an 

 accoiint of which is given in Thomson and Tait's " Natural Philosophy," we may get some 

 knowledge as to the amount of water surface in the unexplored arctic and antarctic 

 regions. 



5. A more accurate knowledge of the tide-currents over the surface of the oceans. 



6. The effect of the wind on the tides from meteorological observations carried on 

 simultaneously. 



The practical results in their bearing on the commerce of Canada, are those that will 

 be valued by the greatest number, and these T purpose now to consider. Tidal observa- 

 tions may be divided into two classes : — 



(1.) Those connected with the prediction of the height of the tide at any given place 

 and given time. 



(2.) Observations on the tidal streams or currents produced by the ebb and flow of 

 the tide. 



Both classes of observations are affected by atmospheric changes ; hence, meteorologi- 

 cal observations are associated with them. 



A knowledge of the time for high and low water at a given place must precede a 

 knowledge of the currents caused by the ebb and flow. It is therefore much to be re- 

 gTetted that Canada is as yet not only unprovided with tide-tables, such as the United 

 States Government publishes yearly for its own Atlantic and Pacific coasts, but has not 

 even possession of the facts on which to base the calculation of such tables. Let us 

 earnestly hope that the memorial which the British Association has addressed to the 

 Government, urging the need of establishing stations to make the requisite observations 

 from which predictions may be subsequently calculated, may be successful. The more 

 difficult problem of tidal currents may then be undertaken. That its solution is possible 

 and that its practical value is great, need not be doubted. To the consideration of it, the 

 remainder of the paper will be given. 



