"I understand," said Fred; "I read about that at school. Every great river has its delta, and some of them are very large. The delta of the Mississippi is more than 300 miles long, and is extending every year. In every ten gallons of water brought down by the river there is one gallon of solid matter, which is deposited in the Gulf of Mexico and must fill it up in time." "Yes," echoed Frank; "and I've read about it, too. If you look on the map of any country where there is a large river, you will see that, it is pushed way out into the sea, and is generally divided into several mouths. This is the case with the Mississippi, the Nile, the Ganges, the Indus, the Mekong, and lots of others. The Amazon has a broad delta, as its mouth is 180 miles wide, and contains an immense island that was formed by the earth brought down from the interior ages and ages ago. What is that?" He pointed directly ahead of the steamer to a spot on the bank where a flag appeared to be waving from a mast. [Illustration: DIAMOND HARBOR.] "I presume it is Diamond Harbor," the Doctor responded, "as it is about time for us to reach it." The speck grew larger and larger, and finally resolved itself into a building like a fortress on a diminutive scale. A few palm-trees waved around it, and, with their glasses, our friends could see that there was an earthwork surrounding the building to protect it from possible assault. From a mast that resembled the cross-trees of a ship two or three flags were waving, and it did not take the boys long to realize that they were nearing a telegraph-station. "You are quite right," said the Doctor, "that is Diamond Harbor, the station whence ships are announced, and before we have passed it the name of our steamer will be flashed to Calcutta. See, they are sending up our flag now." The boys turned and saw that the steamer's flag had been sent to the top of the mast, and was waving in the gentle breeze. They felt that they were once more at the end of a sea-voyage, and Fred remarked that the next thing to arriving at Calcutta was to know that the coming of the ship had been announced there. [Illustration: SCENE ON THE HOOGLY.] They passed the station without stopping, and steamed onward up the Hoogly. Everywhere the banks were the same, and the scenery soon grew monotonous. Forests and groups of palm-trees, and wide spaces devoted to the cultivation of rice or other tropical product, succeeded one another as bend after bend of the river was followed; cattle grazing on the banks were seen at frequent intervals, and occasionally the low walls of a village presented themselves. At last a white object was seen in the distance, and the captain announced that they were practically in sight of Calcutta. CHAPTER XXVI. SIGHTS AND SCENES IN CALCUTTA. [Illustration: RIVER SCENE BELOW CALCUTTA.]