6 On Terrestrial Magnetism. 



pose. Attempts to square the circle have been made without 

 number ; and persons might longer have continued to waste time 

 and talents in this fruitless inquiry, had not Lambert at length 

 demo'nstrated its impossibility in a rational form*. 



In the folloniiig Memoir a general view is given, by its inge- 

 nious author, of all the most material properties that are known 

 on the subject of terrestrial magnetism. Some new and curious 

 mathematical results respecting the inflexions of the magnetic 

 equator, and the lines of no variation and dip, .are derived from 

 the observations made in different parts of the world ; and an 

 hypothesis is proposed for explaining them, which, if not quite 

 correct and satisfactory, at least represents the magnetic actions, 

 and their modifications, in a way that cannot easily be contro- 

 verted, until further discoveries shall have made us better ac- 

 quainted vvith their causes. 



It is earnestly wished that astronomers, travellers, captains of 

 ships, and other gentlemen who possess the necessary instru- 

 naents, would continue these researches, and publish their observa- 

 tions j for it is only by discussing a series of them, made in a 

 great number of places, and continued for a long period of time, 

 that we can expect to arrive at a complete knowledge of the laws 

 of magnetic attraction over the whole surface of the earth. 



Particular requests might be given to the commanders of ves- 

 sels in the service of His Majesty, the East India and other pub- 

 lic and private companies, to determine these data as often as 

 the weather and circumstances will admit. Not only should the 

 dip and variation be taken, but the number of oscillations also 

 of both these needles in ten minutes, commencing at a certain 

 distance from zero, as was begun by Mr. George Graham, and 

 continued by Humboldt and Rossel. It has been proved by 

 the experiments of Mr. Canton, that the temperature of the 

 atmosphere has a sensible influence on the variation of a needle ; 

 and he accounts for the diurnal alterations by means of it ; but 

 v.hether the density, or any of the other modifications of the 

 atmosphere, besides aurora borealis, and lightning, will alter 

 its direction, cannot in the present state of our knowledge be 

 ascertained : it will therefore be proper to note down the baro- 

 meter and thermometer, at least, vvith the latitude and lon- 

 gitude of the place of observation at the time of making each 

 experiment. Considerable difficulty will be met with occasion- 

 ally in taking them at sea, on account of the action of the iron 

 which there is in the ship, the instability of the ivater, and other 

 causes : but perhaps the method now generally adopted, of sus- 



• Memoirs of tlie Uoyal Academy of Sciences of Berlin for 1761. 



pending 



