Historical Introduction 5 



Franklin had Folger plot the course of the Gulf Stream for him and then 

 had a chart engraved and printed by the General Post Office. 



FrankHn (1786, p. 315) believed that the Gulf Stream was caused 



by the accumulation of water on the eastern coast of America 

 between the tropics by the trade winds. It is known that a large 

 piece of water 10 miles broad and generally only 3 feet deep, has, 

 by a strong wind, had its water driven to one side and sustained 

 so as to become 6 feet deep while the windward side was laid dry. 

 This may give some idea of the quantity heaped up by the 

 American coast, and the reason of its running dowTi in a strong 

 current through the islands into the Gulf of Mexico and from 

 thence proceeding along the coasts and banks of Newfoundland 

 w^here it turns off tow^ards and runs down through the Western 

 Islands. 



By the time of Maury, in the middle of the nineteenth century (see 

 Maury, 1859), Frankhn's estimates of the velocities of the Stream were 

 regarded as excessive, but more recent studies tend to confirm them. 

 Franklin did not give any details concerning the edge of the Stream. 

 Starting in 1775, both FrankHn and Charles Blagden (1782), independently, 

 conceived the idea of using the thermometer as an instrument of navi- 

 gation, and each made a series of surface temperature measurements while 

 crossing the Atlantic. On Frankhn's last voyage in 1785 he even attempted 

 to measure subsurface temperatures to a depth of about 100 ft., first with 

 a bottle and later with a cask fitted with a valve at each end. 



A number of subsequent investigators made use of the surface thermo- 

 meter, among them Governor Pownall (1787) and Captain Strickland 

 (1802). It was in this fashion that Captain Strickland discovered a north- 

 easterly extension of the Gulf Stream toward England and Scandinavia. 

 These temperature measurements were not made with any idea of deter- 

 mining the pressure field and geostrophic current, as is done today, but 

 were simply regarded as an indication of the type of water through w^hich 

 the ship was saiUng. 



In passing, the invention of the marine chronometer by John Harrison 

 and its perfection by Thomas Earnshaw should be mentioned. By 1785, 

 accurate chronometers were generally available to ships; this made the 

 determination of longitude at sea at last a possibility and the deter- 

 mination of the set of a current much more exact. Another important 

 oceanographic tool, the drift bottle, was probably first used in 1802, when 

 such a bottle was cast from the Rainbow. The use of drift bottles continued 

 for more than a century, and finally received great impetus at the hands 

 of the Prince of Monaco. 



