Scientific Intelligence. — Meteorology. 181 



ization, which is established at the moment of contact of two bodies 

 susceptible of acting chemically on one another. M. de la Rive also 

 expresses his opinion on the cause of the aurora, which he explains, 

 not by a radiation of the polar magnetism, but by a purely electri- 

 cal action. After examining nearly all recent observations, he be- 

 lieves that he may attribute this phenomenon to the electricity with 

 which the currents of air are charged, that rise from the equatorial 

 regions, and travel in the upper atmosphere towards the poles, where 

 they combine with the negative electricity of the earth, forming, 

 under the influence of the magnetic pole, true luminous arches.-— 

 (American Journal of Science and Arts. vol. xvi. No. 47> 2d Series, 

 p. 274.) _ 



6. " Piroroco''* or Bore that occurs in the Guamd River at 

 Spring Tides. — About thirty miles above Para the piroroco com- 

 mences. There was formerly an island in the river at this point, but it 

 is said to have been completely washed away by the continual action 

 of the bore, which, after passing this place, we rather expected to 

 see, now being the time of the highest tides, though at this season 

 (May) they are not generally high enough to produce it with any 

 force. It came, however, with a sudden rush, a wave travelling 

 rapidly up the stream, and breaking in foam all along the shore and 

 on the shallows. It lifted our canoe, just as a great roiling ocean- 

 wave would do, but, being deep water, did no harm, and was past 

 in an instant, the tide then continuing to flow up with great velocity. 

 The highest tide was now past, so at the next we had no wave ; 

 but the flood began running up instantaneously, and not gradually, 

 as is generally the case. On our way down, I again encoun- 

 tered the " piroroco," when I hardly expected it. We had gone 

 in-shore at a sugar estate, to wait for the tide, when the agent 

 told us we had better put out further into the stream as the piro- 

 roco beat there. Though thinking he only wished to frighten us, 

 we judged it prudent to do as he advised ; and, while we were expect- 

 ing the tide to turn, a great wave came suddenly rushing along, 

 and breaking on the place where our canoe had been at first moored. 

 The wave having passed, the water was as quiet as before, but 

 flowing up with great rapidity. As we proceeded down the river 

 we saw everywhere signs of its devastations in the uprooted 

 trees which lined the shores all along, and the high mud banks 

 where the earth had been washed away. In winter, when the spring 

 tides are highest, the " piroroco" breaks with terrific force, and often 

 sinks and dashes to pieces boats left incautiously in too shallow wa- 

 ter. The ordinary explanations given of this phenomenon are evi- 

 dently incorrect. Here there is no meeting of salt and fresh water, 

 neither is the stream remarkably narrowed where it commences. I 

 collected all the information I could respecting the depth of the 

 river, and the shoals that occur in it. Where the bore first appears 

 there is a shoal across the river, and below that the stream is some- 



