VOL. XLVIII.] PHILOSOPHICAL TRANSACTIONS. 557 



the chart, thus again rendered useful, they presented to the r. s. with an account 

 of the methods used in performing the same. 



Though the most beneficial use of these lines belongs to the sea, yet if they 

 could be extended over the land likewise, the advantages arising would more than 

 compensate the trouble, as will appear by taking a short view of each. 



And first, the use of these lines at sea may be considered either as common to 

 the art of navigating in all large bodies of water; or as particular in some such : 

 the general use being that of steering the true course designed, and finding the 

 ship's true place, as near as may be, by what the mariners call the dead reckoning. 



The particular uses will be best explained by examples : for instance, in the 

 southern parts of the great Atlantic ocean, beginning with the coast of Brazil 

 and Patagonia, and proceeding to the south of the Cape of Good Hope into 

 the Indian ocean, as far as the common tracks of our East-India ships extend, 

 the variation lines have appeared to be, for the most part, directed northward 

 and southward : whence, in most places of that great body of waters, if the lati- 

 tude and variation be found by celestial obsei-vations, the longitude will be ob- 

 tained by the lines on the chart ; the great usefulness of which has been attested 

 to the writers, by many persons who have, successfully to themselves, practically 

 applied the last constructed chart, to correct their dead-reckoning on that long 

 passage. 



Indeed, where the variation lines run nearly eastward and westward, as has 

 appeared in the Atlantic ocean, from the west coast of Europe to the east coast 

 of North America, no assistance toward obtaining the longitude can be derived 

 from them ; but as it frequently happens, within those limits, that meridian ob- 

 servations, for determining the latitude, cannot be obtained, especially about 

 Newfoundland ; then, if a good observation of the variation can be taken, at 

 any time of the day, the latitude may be nearly ascertained by the lines on the 

 chart. 



Secondly, the advantages that will arise by extending the variation lines over 

 the land, as well as sea, will be the confirmation of those drawn over the waters ; 

 the continuation of which, from sea to sea, will be thus conspicuous, and we 

 hall be enabled to judge better of their nature, properties and causes; and if the 

 same can be extended over all the parts of the known world, the eye will be 

 presented, at one view, with the different degrees of attraction, with which all 

 the parts of this great magnet are endued, at the time when such lines are 

 drawn ; this the writers would have attempted to do, in the year 1 744, if they 

 could have procured a sufl^cient number of observations for that purpose ; but 

 though they frequently advertised their request, in the public papers, no assist- 

 ance was obtained. 



As the writers have by experience found, that the proper period for re-examin- 



