[shaw-henry] 



SIMPLE PERIODIC CURVES 



141 



But in music the overtones are actual vibrations, such as the fifth 

 or the octave above the primary tone; whereas in the case of the 

 estuary tides the periods are quite arbitrary, and do not correspond 

 individually to separate physical factors. These overtides have periods of 

 one-third, one-fourth, one-sixth, and one-eighth of the lunar day, one- 

 fourth of the solar day, and in the notation, the corresponding constants 

 are Ms, M 4, Me and Ms, with S4 for the solar term. The unequal form 

 of an estuary tide gives these terms quite a large amplitude, relatively 

 to such primary components of the tide as the lunar semi-diurnal and 

 the solar semi-diurnal; as the following table will show, in which a 

 comparison is given for a few leading harbours of the estuary type. As 

 to the St. Lawrence tide stations, Father Point is in the open estuary 

 and Quebec is not far from the maximum amplitude of the tide. 



Overtides Compared with Leading Semi-Diurnal Components 

 (Semi-amplitudes of sinusoids, in feet) 



An entirely different method of dealing with the problem by an 

 oblique projection of the open-water or original oceanic tide curve will 

 now be discussed. It was suggested to the Superintendent of Tidal 

 Surveys by the shadow of suspended chains on the ground; as these 

 approximate to half-sinusoids, and the oblique shadow line was 

 strikingly like an estuary tide curve. By forming a sinusoid with a 

 bent wire, it was found that the change in the form of the tide curve 

 in its progress up an estuary, could be represented by a more and more 

 oblique projection of the symmetrical sinusoid of the original oceanic 

 tide curve. 



§2. The Method of Projection. The photograph of the apparatus 

 shown in Fig. 7, and discussed again in a subsequent paragraph, 

 illustrates the method of projection. A graph cut out of stiff card- 



