i84 THE POPULAR SCIENCE MONTHLY. 



would flow to the westward. For two reasons, however, it would 

 be confined to equatorial regions : first, the centrifugal force there 

 is greatest ; and, second, the meridians converge as we near the 

 poles. 



This second reason will appear evident if we suppose a body of 

 water of five degrees area and any depth to set out from the equator 

 toward either j)ole. At each remove it would find the linear dimen- 

 sions of a degree smaller. The depth remaining constant, its volume 

 would be too great for an area of five degrees square in latitude 30°, 

 still more so for one in latitude 60°, and so on. This constant crowd- 

 ing in extra-tropical zones would therefore constitute an opposing 

 force sufiicient to confine the flow of water to a zone where its volume 

 would undergo little or no compression — that is, in the vicinity of the 

 equator. 



Arriving, then, at the eastern shores of the continents to the west- 

 ward of those from which it started — at the North and South Amer- 

 ican shores, for instance, having started from Europe and Africa — and 

 being banked up by constantly arriving volumes of water, it would be 

 forced to the northward and to the southward along the coast-line of 

 each continent ; it would then flow to the eastward in high latitudes 

 until reaching the western shores of the continents from which it 

 started, where, owing to the divergence of the meridians toward the 

 equator and the greater centrifugal force at that parallel, it would 

 flow from the north and from the south along the shore-lines of the 

 continents until reaching the intertropical zone, where it would again 

 start westward on its circuit. 



Imagine this system of circulation once set up, and nothing is more 

 natural than that it should continue while the earth revolves ; indeed, 

 a glance at any current chart of the world will suffice to show the 

 force of this reasoning. 



It will now be seen how important a part the thermometer and hy- 

 drometer play in the discovery of oceanic currents : by the former a 

 difference of temperature, and by the latter a difference of density, is 

 quickly detected ; and, if a decided difference of either kind is found, 

 a permanent current may be fairly inferred. A consideration of the 

 winds, whether an accidental gale, the constant trades, or the seasonal 

 monsoon, may lead us to deduce intelligently whether a set that may 

 have been experienced for days is a temiiorary surface-flow or a per- 

 tnanent current. So, also, keeping in view the range of the barometer 

 for a few days — the locality and amount of its rise or fall — may assist 

 in deciding whether a certain set be due to its extreme range or not. 

 A consideration of the rotation of the earth is of assistance only in 

 determining the general direction of the great ocean-currents. 



A few other entries of the log-book require a passing notice. The 

 direction of the wind is indicated by a vane in connection with a 

 compass, and its force is estimated according to the following scale : 



