INTRODUCTIOX. 7^ 



recognising the pole on its rising again to the sur- 

 face of the ocean, as it may easily be carried to a 

 great distance by currents ; but this objection has 

 been attempted to be obviated by means of flags 

 fastened to it, {Jiamme de rec'oimoissance.^ How- 

 ever, no experiment, at least none that is satisfac- 

 tory, has hitherto been made with such a machine. 

 AVe refer the reader to the proposals of Greenstreet, 

 in the Repository of Arts and Manufactures, and 

 to an essay on the subject, by Ciprian Luiscius. 



Among the peculiar properties of the sea, is, 

 above all, its saltness. It might be inconvenient 

 to examine this by chemical analyses on board a 

 ship ; but as the strength of the solution of salt in 

 water increases exactly with its weight, the deter- 

 mination of the specific gravity of the sea-water 

 affords a convenient means of finding the re- 

 lative quantity of sea-salt contained in it. For 

 this purpose, it is the best way to employ an are- 

 ometer, which is plunged in sea-water in a vessel. 

 Such an instrument is best for use, when it is so 

 contrived, that with a constant weiglit fastened to it, 

 it sinks, at a certain temperature, in pure, distilled 

 water to the mark : the weights to be added must 

 be exact aliquot parts of the weights of the whole 

 areometer. We have then only to count at each 

 observation, the value of the weights added, to ob- 

 tain the specific graA'ity of the sea-water. As, 

 however, the sea-water becomes 0,00034 = ^^ 

 lighter for every degree of heat, by Reaumur's 



