CHARTING THE RIVER 113 



uring the surface stream. On September 19, 1492, while be- 

 calmed in the southwesterly current, he sounded with the 

 deep sea lead line. It is not surprising that he recorded no 

 bottom at 200 fathoms, since on modern charts the sounding 

 nearest his probable position is 2,290 fathoms, or over 12,000 

 feet deeper than the length of his line. Nevertheless, at 200 

 fathoms the water in this locality is relatively motionless. As a 

 result the weighted end of the line entering the still water 

 below was held back by friction drag while the ship itself, 

 being becalmed, drifted with the surface current. Conse- 

 quently the lead line was pulled a considerable distance from 

 the vertical. If Columbus had had sufficient knowledge of 

 physics he could have calculated from this the actual rate of 

 flow at the surface in relation to the deeper levels. All he actu- 

 ally did was to note the presence of a current carrying him 

 southwest. 



Two important instruments, the modern sextant and the 

 chronometer, came into use in the latter part of the eighteenth 

 century and provided navigators and the oceanographers who 

 came later with tools of greater accuracy for their difficult task 

 of measuring the Atlantic current. The ship's chronometer, 

 which became generally available about 1785, was invented by 

 John Harrison and later developed by Thomas Earnshaw. It 

 came into use during Franklin's day, and undoubtedly pro- 

 vided a great stimulus to the accurate plotting of ocean cur- 

 rents. Before this it had been impossible to determine the 

 longitude accurately when out of sight of land, because this 

 depends on a very accurate measurement of time as well as on 

 knowing the astronomical position of heavenly bodies. 



At the midspring or midfall equinoctials, when the sun is 

 directly over the equator, the latitude is easily measured from 

 the sun's noontime altitude or angular height above the hori- 

 zon, by subtracting this from 90°. At other times of the year, 

 provided the declination of the sun or its distance from the 



