286 DEPARTMENT OF THE NAVAL f^ERTICE 



Now table 5 contains values of 2 A not only for the surface of the sea, but also for 

 isobars at deeper levels. The topography of these is then obtained in the same manner 

 as that of the surface. We can thus, by the process here described, ascertain in the 

 slope of the isobaric surfaces, and so arrive at the distribution of pressure throughout 

 the entire portion of sea from which observations are available. 



From the slope of the isobars, it is easy to ascertain the acceleration of the water. 

 The most simple formulation of the relation between these two magnitudes is that given 

 by Dr. J. Hann, viz., that the acceleration of the water is equal to that component for 

 acceleration of gravity which falls in the isobar surface. Popularly speaking, we may 

 say that the water slides down the sloping isobar surfaces, and that this sliding move- 

 ment is practically frictionless. 



This again, however, is as much as to say that the sea-water in such respect may 

 be likened to the water of a river, and we can also apply the well-known laws for the 

 flow of river water to the currents in the sea. As an example we may take the Gulf 

 Stream. In this case, 



2 A = 150000 

 On inserting this value in (18) we find that the surface of the sea lies about 1^ metres 

 higher in the Tropics than in the Arctic ocean. In other words, the Gulf Stream may 

 be regarded as a great river, flowing down from a higher to a lower level. 



The Gulf Stream carries 25,000,000 tons of water per second. This, with a fall 

 of 1-5 metres, amounts to a force of 500,000,000 horse-power, which is the amount of 

 energy expended in the propulsion of the Gulf Stream. It is utilized to overcome 

 friction, and is thus converted into heat. 



If we now insert 5A=10880 in (18) we find that the surface of the sea during 

 the summer of 1915 was 11-1 cm. higher at Station X 31 in the gulf of St. Lawrence 

 than at station XIII 38 south of Nova Scotia. And as the Gaspe current carries 

 645,000 tons of water per second, its production of energy amounts to 955,000 horse- 

 power, this force being latilized for its propulsion, and converted into heat by the 

 internal friction of the water. 



By thus directly comparing the ocean currents with rivers, we find it easier to 

 comprehend the nature of the former, and the processes which take place therein. In 

 a river, the slope of the river-bed occasions a continual transformation of potential 

 energy into motive power. So also with ocean currents. In these, the slope of the sea's 

 surface and of the isobars will represent a certain quantity of potential energy, which 

 is gradually transformed into motive power, and thus maintains the movement of the 

 current. And we have just seen, how the magnitude of this transformation of energy 

 may be expressed in horse-power. 



It is thus easy to follow the transformation of energy in the sea. The heat 

 derived from the rays of the sun in the Tropics is first converted into potential energy, 

 which again is then transformed into motive power, setting in motion currents which 

 transport the warmer water to higher latitudes, where the heat is again given off from 

 the sea. By periodical oscillations in the movement of the sea-water, the energy repre- 

 sented is continually being converted from potential force to motive power, and vice 

 versa. The potential energy, however, after first being transformed into motive power, 

 is thence again converted by the internal friction of the water into heat. We have thus 

 the following system of transformation of energy in the sea : — 



HEAT ._^ POTENTIAL ENERGY > MOTIVE POWER 5, MEAT. 



This applies to all ocean currents. Thus the water in the St. Lawrence river, for 

 instance, is due to the heating of the sea-water by the sun's rays, viz., firstly evapora- 

 tion, then raining. The mixing of this fresh water with sea-water in the gulf of St. 

 Lawrence gives rise to the potential energy which propels the Gaspe current. And 

 finally, the motive power of this latter is trausformed, by the internal friction of the 



