It was not long after the steamer passed the mouth of the San Carlos before the meaning of the rafts became apparent. On the 23d of December, while the company stationed at the mouth of the Serapaqui were at dinner, they were surprised by a body of Costa Ricans about 120 strong, led on by a man named Spencer. When Thompson, who commanded at the Serapaqui, was attacked by Spencer, he had no sentries posted, and the arms of the men were at some little distance from the place where they were dining. Spencer had got to the rear of the American camp, and by placing a soldier in the top of a tree he was able to know accurately the state of Thompson’s camp. The surprise was complete, and most of the Americans were either killed or wounded. Thompson was made prisoner; his conduct and courage were praised by the Costa Ricans, and he himself was liberated soon after being taken to San Juan del Norte. Well might the Costa Ricans afford to laud Thompson, for it was his criminal neglect of duty which enabled them to get possession of the point at the mouth of the Serapaqui, and thereby secured the success of their subsequent operations. Spencer had marched with his Costa Ricans from San José to a point on the San Carlos river, some miles above its mouth, and had thence floated his men on rafts down to the mouth of the Serapaqui. In addition to the force which attacked Thompson on the 23d, a large body of soldiers had been marched to the San Carlos, under the orders of General José Joaquin Mora, brother of the President, Juan Rafael Mora, and commander-in-chief of the Costa Rican army. The march was very difficult from the nature of the country through which it was made, the region between San José and the San Carlos being entirely uninhabited, and wholly destitute of subsistence. The road over which Mora marched was a mere trail, and his soldiers had at times to cut their way with machetes through the thick undergrowth. The results of the march depended wholly on the success of Spencer’s efforts to get possession of the river San Juan and of the boats plying on it, and Spencer, as we have seen, owed his first and most important success to the gross and criminal negligence of Thompson at the Serapaqui.