Magnet'ical Ohfervations and Eftqtitries, og 



have Hkewife remarked, that two points, of which the aflion is very ftrong, are joined by- 

 rocks which do not exert the lead attradion. The chemical analyfis of thefe compounds 

 affords the fame refults; and it would be no iefs difficult to difcover any difference of ag- 

 gregation between them, than between iron which has received the touch, and other iron 

 which had never acquired the magnetic power. 



On this occafion a queftion prefents itfelf wlilch cannot be refolved in Iefs than half a 

 century. The tables founded on the ohfervations of Picard, La Hire, Maraldi, Caffini, and 

 Le Monnier, fliew that the needle has declined fince 1660 towards the weft; and that this 

 declination continues to increafe, though the ofcillations caufed by the heats of the fouth, 

 and the temperature of the feafons, often produce a retrograde courfe. If the magnetic 

 axis of our mountain were aflronomically determined by the culmination of the ftars, 

 •whether its direftion would remain the fame until the year 1850, or whether its fouth-pole 

 would turn towards the weft, in conneflion with the variation of the magnetic needle ? 

 From our profound ignorance of the caufes of the declination, as well as of moft geological 

 phenomena, it is not in our power to refolve fo complicated a problem. 



Other ohfervations equally intereding may be made on the identity of magnetic forces. 

 I have difcovered a mafs of rocks which alfefl the needle at the diftance of twenty-two feet. 

 With an apparatus fimilar to the magnetometer of M. de Sauffure, we might obferve whether 

 the intenfity of the forces of magnetic aflion remains the fame in winter and in fummer; 

 whether it be ftronger in the morning, at noon, at the folttices, during the aurora borealis 

 or in an atmofphere loaded with eleftric fluid } I fuppofe that thefe fame rocks might a£l 

 on the needle fometimes at 16, and fometimes at 28 feet diftance. 



It has been obferved, that metals expofed to the air gradually imbibe the magnetic fluid. 

 A flight oxidation of the iron feems to favour this effeft. I have myfelf obferved, that in 

 a magnetic bed of iron thofe parts only which were in contaft with the air afl^efted the 

 needle. This phenomenon is confidered as the efl^eft of atmofpheric eledricity. I am 

 aware that lightning converts a bar of iron into a magnet ; that the difcharge of the 

 Lcyden vial fometimes increafes the intenfity of magnetic forces ; but I do not fee why the 

 atmofpheric eledlricity (hould aft fimply on the external furface of a bed of magnetic iron, 

 which is a good condu£lor of the eledric fluid. Does not the oxygene of the atmofphere 

 rather adl a part in this operation? Without wandering in the fphere of probabilities, I have 

 chofen to adhere to enquiries refpefting faiSs. I have obferved the rocks which were 

 covered with turf, from which I detached pieces that had not been in contaft with the air. 

 1 found that the magnetic force was conftantly the fame. 



The mountains of the Harz prefent a granite rock called the fchnarcher, which is elevated 

 in the form of a tower, or broken pyramid. This granite likewife affeds the needle ; but it 

 acts only in the mafs, and in a (Ingle band or perpendicular vein. Detached pieces (hew no 

 aftion upon the needle. It is to Mr. de Trebra, celebrated for his refearches concerning 

 the internal parts of mountains, that we are indebted for this important difcovery. 



.Some philofophcrs pretend that the fchnarcher contain in their bowels a mafs of magnetic 

 iron ; others prcfumc that a ftroke of lightning has caufed the magnetic vein in thefe 

 mountains. 



The nature of the rocks which I have the honour to prefent to your notice in this paper 

 does not admit of fimilar explanations. The ferpentine not only a^s in a mafs, in its na- 



O 2 tural 



