Sir,—I have the honour to report that we left Honolulu on the 11th of August, and on the 12th we sounded in 2050 fathoms, and took a series of temperature observations between the islands of Oahu and Hawaii. At night a crimson reflection was visible in the position of the top of Mauna Loa; but as we understood that the side crater of Kilauea only was in eruption, we supposed that that was some great conflagration of forest on the flank of the mountain. All the following day we steamed against a head wind along the rugged and picturesque coast of Hawaii—headlands of lava and volcanic ash separated by deep wooded ravines or “gulches,” with a rapid hill-stream running in the bottom of each. The top of Mauna Loa was covered with clouds during the day; but at night a splendid crimson glare hung over the mountain, and lit up the clouds to a wide radius, and it became evident that we were fortunate enough, to see one of the rare eruptions of the summit crater. Early on Saturday we cast anchor in Hilo Bay; and a party of us made arrangements to start as early as possible for the volcano of Kilauea thirty miles distant, intending, if the eruption of the summit crater continued and if we found it practicable, to push on to the top of the mountain. We left Hilo in the afternoon, taking magnetic and photographic instruments with us, and rode all night through the forest and over the lava-flows, reaching the rest-house at the side of the crater of Kilauea early in the morning. During the early part of the night the light from the summit crater was very brilliant, but it began to pale soon after midnight; and when we arrived at Kilauea we found that the column of smoke at the top of the mountain had almost entirely disappeared, and that the eruption was virtually over. This was no great disappointment to us; for we had learned that the ascent of the upper peak could not be undertaken with reasonable safety without much greater preparation in the way of provisions and warm clothing than we had had time to make, and we should have been obliged to give up the idea in any case. In the afternoon we went down into the crater and walked for about three miles over the nearly level lava of the eruption of 1868; we then clambered up to the ridge overlooking the two liquid lava-lakes, which have remained nearly in the same condition since 1868. We were greatly struck with the fluidity of the melted lava, which washed about in the basins with very much the appearance and sound of water. The night was perfectly still; and it had a most singular effect to see the two glowing lakes tossing like the sea in a storm, and a red surf dashing against the encircling rocks and springing forty or fifty feet into the air in wreaths of fiery spray. Lieutenant Bromley and I recrossed the lava-bed after nightfall, and our guide missed the ordinary path. In several places on either side of us the surface of the lava was glowing of a dull red, and we could see through the cracks the crust red-hot a couple of inches under our feet, and the liquid lava flowing beneath at the depth of about half a yard. Observations were made by Lieutenant Bromley in the crater and on its rim with the “Fox” dip-circle for inclination and intensity, and with the prismatic compass for declination. Several excellent photographs of the crater were taken during the day. We returned to the ship on the evening of the 16th, and on the 19th we left Hilo and proceeded under sail for Tahiti, 2270 miles distant.