TIDES AND CURRENTS IN BOSTON HARBOR \)\) 



In its rising and falling the tide does not move at a uniform rate. From 

 low water the tide begins rising, very slowly at first, but at a constantly increas- 

 ing rate for about three hours, when the rate of rise is a maximum. The rise 

 then continues at a constantly decreasing rate for the following three hours, 

 when high water is reached and the rise ceases. The falling tide behaves in a 

 similar manner, the rate of fall being least immediately after high water, but 

 increasing constantly for about three hours, when it is at a maximum, and 

 then decreasing for a period of three hours till low water is reached. 



The rate of rise and fall and other characteristics of the tide may best be 

 studied by representing the rise and fall graphically. This may be done by 

 reading the height of the tide at regular intervals on a fixed vertical staff gradu- 

 ated to feet and tenths and plotting these heights to a suitable scale on cross- 

 section paper and drawing a smooth curve through these points. A more con- 

 venient method is to make use of an automatic tide gauge by means of which 

 the rise and fall of the tide is recorded on a sheet of paper as a continuous curve 

 drawn to a suitable scale. Figure A shows a tide curve for Foit Hamilton, 

 N. Y., for July 4, 1922. 



In Figure A the figures from to 24, increasing from left to right, represent 

 the hours of the day beginning with midnight. Numbering the hours con« 



Fig. a.— Tide curve for Fort HamQton, N. Y., July 4, 1922 



secutively to 24 eliminates all uncertainty as to whether morning or afternoon 

 is meant and has the further advantage of great convenience in computation. 

 The figures on the left, increasing upward from 2.0 to 9.0 represent the height 

 of the tide in feet as referred to a fixed vertical staff. The tide curve presents 

 the well-known form of the sine or cosine curve. 



The difference in height between a high water and a preceding or following 

 low water is known as the "range of tide" or "range." The average difference 

 in the heights of high and low water at any given place is called the mean range. 



THE TIDE-PRODUCING FORCES 



The intensity with which the sun (or moon) attracts a particle of matter on 

 the earth varies inversely as the square of the distance. For the solid earth as 

 a whole the distance is obviously to be measured from the center of the earth, 

 since that is the center of mass of the whole body. But the waters of the earth, 

 which may be considered as lying on the surface of the earth, are on the one 

 side of the earth nearer to the heavenly bodies and on the other side farther away 

 than the center of the earth. The attraction of sun or moon for the waters 

 of the ocean is thus different in intensity from the attraction for the solid earth 

 as a whole, and these differences of attrfiction give rise to the forces that cause 

 the ocean waters to move relative to the solid earth and bring about the tides. 

 These forces are called the tide-producing forces. 



