BOUGUER's voyage to PERU. 



31 i 



augments or diminifhes according as thefe rays are found united in a greater or lefler 

 fpace. When the rays are divergent, the force of light decreafes ; and it continues to 

 do fo, at leaft until, by the failing of it upon a convex glafs, or a concave mirror, 

 the divergency is changed to a convergency : then the force of light increafes, although 

 received at a greater diflance from the luminous body. So it Ihould be v/ith regard to 

 the magnetic virtue. The diredions, according to which this power operates, are a 

 fpecies of meridians, and they are at the greateft poflible diftance from each other in 

 the environs of the equator ; it is there, then, the magnetic power fhould be the weakeft. 

 But if we advance into either of the other hemifpheres, we are not to imagine that it is 

 the effed: of the pole to which we may be approaching, that folely operates to its aug- 

 mentation ; it will be alfo the effeft of the other pole ; fince thefe diredlions are the 

 fame as the rays of light, which, from being divergent, become convergent. Thofe 

 direftions which are the farthefl feparated from each other towards the equator, mu- 

 tually draw to each other as they go forward. According to this, the force of the 

 auftral magnetic pole as felt at Paris, fliould be fenfibly equal to that we fhould experi- 

 ence from the part of the fame pole, if we were at the fame diftance from the equator 

 on the other fide. Thus, generally fpeaking, and fetting afide every confideration 

 upon which one might infift, it matters not on what place of the earth we ftand ; it 

 fignifies nothing, that it is or is not an equal diftance from the two poles ; we (hall 

 always feel the a6lion of one pole as powerfully as the other. It is true, that the force 

 of each pole will be greater or lefs, but the two will neverthelefs be always equal, which 

 alfo my obfervations confirm. The refiftance of the air will apparently introduce fome 

 -difference between the two aftions, if the magnetic matter creeps upon the furface of 

 the ground, and if it has a long voyage to make through the grofs air we breathe. But 

 the inclining of the needles marks out the route taken by the magnetic matter ; and this 

 route varies little from a vertical one below, which demonftrates, that the magnetic 

 matter has prefently pafTed through the groffer air, and that its paffage through the 

 higher region is made above the denfer part of the atmofphere. 



My readers will, doubtlefs, not difapprove, that, in giving him an account of thefe 

 obfervations, I have condu6ted him from Peru to Europe, to inftantly carry him back 

 towards the middle of the torrid zone. When I got out of the Cordelier, I had no 

 reafon to doubt, that, if the country was low enough, I fhould find it nearly the fame 

 in quality as that on the other fide of the double chain of mountains. In the mean- 

 time, I was ftruck at the firft view, with the difference in many refpefts. Plata is of 

 moderate elevation : the mercury in the barometer ftood exaftly at twenty-five inches ; 

 and at Honda at twenty-feven inches and five lines and three-quarters. The upper 

 grounds are all ftony, and the country naked. The environs of Plata, which is four 

 or five leagues to the weft of the river Magdalene, are tolerably peopled ; the reft but 

 thinly ; and the places or towns, Honda and Mompox excepted, towards the fea, are 

 of little confideration. Mompox is ornamented with a very fine quay, which they have 

 been obliged to raife high, by reafon of the fudden fwells to which the river is liable ; 

 for, although it is very wide, it rifes every year, at the beginning of December, to 

 twelve or thirteen feet. It has its courfe between rocks, and upon fand, as far as the 

 midway between Honda and Mompox; but it experiences, below, much the fame 

 change as the interior of the Cordelier. It rolls it waters over flime j its fine fhores 

 are converted below almoft entirely into moraffes, fome of which fpread very wide 

 around. 



One fingular circumftance has frequently drawn my attention throughout all thefe 

 countries ; and that is, all the mountains near which I journeyed, and which are at the 



1 2 feafe. 



