^lO BOUGUERS VOYAGE TO PERU. 



fully by the north pole of the earth, it will communicate by degrees a motion to the 

 copper needle, and both in moving towards the north, will fix themfelves upon the 

 magnetic meridian ; fo that the two needles will form a ftraight line. 



Every thing being prepared, I made not one only, but twenty or thirty experiments 

 at Quito ; and I made them with the more care, as I was prejudiced in favour of the 

 ppinion I propofed to myfelf to verify or confirm : but all I could do, the copper 

 needle was never moved by the other, and remained always ftationary. Neither could 

 I attribute its repofe to the fridion of the pivot, for when I put the two needles toge- 

 ther, they took very properly the diredion given to them by the loadftone. I alfo, 

 fometimes, put the copper needle in motion, while the other was left at perfe61: liberty, 

 and the firft remained always indifferently fixed in every direction. From hence it 

 may be naturally concluded that the two magnetic poles of the earth, which perhaps 

 refiilt themfelves from the complication of many others, have fenfibly the fame power. 

 Our ordinary magnetic needles, when they take a certain direction, yield to this power ; 

 and they cannot advance, literally, in their length, becaufe they are kept back by their 

 centre, but as that I made ufe of in my experiments was in every refpeft moveable, 

 and neverthelefs it made no advance neither to the north nor fouth, it was a demonftra- 

 tion not to be difputed, that one of its extremities had no more tendency towards one 

 of the poles, than its other extremity to the oppofite. 



The equality between the abfolute forces being eftablifhed, although contrary to my 

 expectation, it remained to me to examine the relative powers ; I mean to fay the per- 

 ceptible power of the part of the pole from which' we may be receding, and that of the 

 other which Ihould naturally encreafe in proportion as we draw near it, and this I 

 could do with much facility in my journey, by a repetition of obfervations in places 

 diverfely remote from the equator. I made three trials on my way, the third at Por- 

 quera, a little town low on the river Magdalene, and three leagues from its mouth ; but 

 the fuccefs of thefe experiments was fimilar to that at Quito. The centre of gravity of 

 the magnetic needle, although moveable, remained always at reft while placed upon 

 the magnetic meridian. I now began to think I had not advanced far enough north- 

 ward, notwithftanding I was already eleven degrees removed from the equator, which 

 made twenty degrees of difference between the diftances of the two oppofite poles. 

 At length on my arrival in France, I again repeated the experiment, but with no other 

 fuccefs. As I was apprehenfive I had not taken all the precautions necelfary, it did 

 not fatisfy me to proceed in the fame manner, I had recourfe to another expedient by 

 which I could not fail to perceive the moft minute inequality. I fufpended a magnetic 

 needle from its center of gravity, with a number of hairs five or fix feet in length ; 

 this new line would not fall vertically, but incline below a little to the north, if it 

 is true that the boreal magnetic pole to which we are moft adjacent, ads with a 

 greater force than its oppofite ; and a digrelTion of five feconds, or a difference in the 

 power not more than a forty thoufandth part of the weight of the needle, would have 

 been manifeft to me. But by all the attention I was competent to, I was not able to 

 difcern any horizontal tendency which compofed itfelf with the weight, or altered at 

 all the direction ; it always appeared to me that the hair hung vertically, and that while 

 the needle pointed north and fouth, it made not the leaft effort in the direction of its 

 length, to move towards either of the poles. 



It is only fince my return, and reflecting more on the fubjeCt, that I have at length 

 conceived the reafon for the conftant perfeCt equality, which appeared to me fo extra- 

 ordinary, always prevailing between the attractive powers of the two poles. The di- 

 rection of the magnetic efflux may be compared to the rays of light, whofe power 



*" augments 



