The rapids of Machuca, which derive their name from Capt. Diego Machuca, who explored this river in 1529, are the first and most formidable on the river. The bed of the stream, for nearly a mile, is full of rocks and stones, between which the water rushes with great force. The boats, in ascending, are kept close in the right shore, and are poled up, slowly and with great difficulty. In descending they are often kept near the middle of the stream, down which they come, glancing between the rocks with the rapidity of an arrow. In descending, in June, 1850, my bongo, which obeyed the rudder very imperfectly, struck with immense force, and got jammed between the rocks, with its broadside to the current, where we remained for thirty hours, until literally dragged out by the united crews of six boats, after half a day of incessant labor. The boat was of great strength, or it must inevitably have gone to pieces. Such accidents are not of frequent occurrence, as the marineros are extremely expert in the management of their bongos. We were four hours in passing the Machuca. From thence to the Rapides del Mico and los Valos, the current is strong, but the channel is free. These rapids are short, and less difficult to overcome than those of Machuca. It is nevertheless a slow and laborious task to make their ascent; and until they are improved by art, they must always be great obstacles to the navigation of the river. At present the steamer “Orus,” sent out by the “American Atlantic and Pacific Ship Canal Company,” lies a wreck on the rocks of Machuca. [Illustration: “CASTILLO VIEJO,” OR OLD FORT OF SAN JUAN.—1849.] On the morning of the 17th of June we made the Rapides del Castillo, commanded by the ancient fort of San Juan, now called the Castillo Viejo, “Old Castle.” We had looked forward to our arrival here with great interest, not less on account of the historical associations connected with the place, than because, from hence to the lake, the passage is quick and comparatively easy. The morning was wet and gloomy, and altogether the most forbidding of any we had yet encountered, hardly excepting that on which we had made the coast, in the execrable little Francis. I nevertheless put on my water-proof poncho, and took my seat by the side of Pedro, on the _pineta_. A league below the fort we passed the island of Bartola, on which, beneath the dense verdure, we could discover traces of the ancient advance works of the fortress. It was here the English buried their men who were killed, or died of disease during the memorable but fruitless expedition against Nicaragua, in 1780, under the command of Colonel Polson, and Captain, afterwards Lord, Nelson. This island was carried by Nelson, who here distinguished himself for the first time.