CANADIAN FISHERIES EXPEDITION, lOlJf-lS 23t 



to the east of there, and still shallower in the Cabot s.trait. It is this variation in 

 depths which gives the current its forward movement. 



The difference of level in the separating surfaces in the sea is of the same impor- 

 tance to the movement of sea-water as the varying level of the surface of a river to 

 the movement of the latter. The force impelling a sea current may be calculated 

 from the slope of the separating surface in the same manner in which the force of 

 a river's current is calculated from the slope of its surface water. In the case of 

 marine currents, however, we have to take into consideration the diiference in density 

 Pi — p.. between the two layers, instead of reckoning merely with the full density of 

 the sea-water. The same thing should, as a matter of fact, be done in the case of 

 a river ; the density of the air above the water, howe-s^er, is so slight as to be negligible 

 in comparison with that of the river water. Save for this, the methods of calcula- 

 tion would be exactly identical for both river and sea currents. 



We can also, if preferred, reduce the difference in level of the separating sur- 

 faces correspondingly, multiplying them by 



Pi P^ 



P 



and then reckon with the entire: 



mass of the sea current. 



This reduction of density naturally tends to diminish considerably the effect 

 of a marine current; this is, however, great enough, owing to the enormous mass of 

 water involved in the movement. Thus for instance, the difference in level of the 

 separating surface in the course of the Grulf stream, amounting to 400 m., is reduced 

 to only 1-5 m. But as the Gulf stream carries 25,000,000 tons of sea-water per 

 second, this slight waterfall yet supplies a force equivalent to 500,000,000 horse-power, 

 which is sufficient to overcome the friction and keep the current in motion. 



From measurements of the depth and velocity of the Gaspe and Cabot currents we 

 may calculate, in a similar way, the force and amount of energy which serves to main- 

 tain the movement of this current, so important for the hydrographical conditions in 

 the Gulf of St. Lawrence. 



Obviously, this simple quantitative method is enormously valuable in determining 

 the causes, features and effects of a marine current. 



The propulsion of the currents is thus the most important work performed by the 

 Archimedean forces in the sea. They have, however, also another function of import- 

 ance here. When an external force acts in the water, the latter is at first moved in the 

 direction whither that force is tending, vide fig. 6. This gives rise to Archimedean 

 forces tending in the opposite direction. As long as this displacement of the water 

 continues the strength of the opposing Archimedean forces continually increases. 

 When this has reached a power equal to that of the external force in operation, a state 

 of equilibrium is attained, and the movement of the water ceases. From the Archi- 

 medean forces, therefore, we can ascertain in this case at once the direction and 

 magnitude of the external force. 



We can thus, from the form of the isosteric surfaces, discover what forces are- 

 acting upon the water. Plates X and XI, showing the shape of the isosteric surfaces 

 in the Canadian Atlantic waters during the spring and summer of 1915, thus give us 

 at the same time an idea of the forces then acting upon the water there. We see that 

 the isosteric surfaces slope from out to sea inwards towards Cabot strait, reaching 

 there a considerable depth. This shows, that some force is at work, tending in towards 

 the gulf of St. Lawrence. It is the deflecting force of the earth's rotation, which 

 presses the Labrador current to the right, in towards Cabot strait, and causes the 

 isosteric surfaces to slope as we have seen. This obliquity, however, again gives rise 

 to Archimedean forces in the water, pressing outwards, and opposing the inflow of the 

 Labrador current into the gulf. And from the magnitude of the Archimedean forces 

 here we can, as will later be seen, ascertain the velocity of the Labrador current in this 



