From my study of the clouds I arrived at the conclusion that there were three prevailing air-currents on the summit of Mauna Loa:

(1) The updraught of warm moist air on the south and south-west slopes of the mountain.

(2) The north-east trade wind, the upper limit of this air-current being probably not far above the summit.

(3) An upper air-current from the south-east (E.S.E.-S.S.E.), which, from the velocity of the clouds it carried, was often probably not over a couple of thousand feet above the summit. It may be observed that on the coast at the base of the southern slope of the mountain in the middle of September, when the wind was N.E. and carried the lower clouds with it, the upper clouds were, on several occasions, noticed travelling in the opposite direction, namely, from the south.

The volcano was quiescent during my visit and could have exercised but little influence on the air-currents.

The Shadow of the Mountain.—Every morning and evening, in clear weather, for about twenty minutes after sunrise and before sunset, the shadow of the mountain was thrown back against the sky of the opposite horizon. It seemed as if some Titanic brush, at work in the sky far away, had painted in the profile of the mountain with a very uncanny blue. At sunset the peak was the last to disappear. Commodore Wilkes, who only records it once, namely, at sunset on the 1st of January, describes it as “a beautiful appearance of the shadow of the mountain projected on the eastern sky ... as distinct as possible, its vast dome seemed to rest on the distant horizon.” This phenomenon is, of course, well known in the case of other isolated mountains. According to Murray’s Handbook of Southern Italy (1892), the correct thing for a visitor to Stromboli is to make an early ascent of the cone to observe “the very curious triangular shadow of the mountain cast by the rising sun upon the sea.” Unfortunately I neglected my opportunity when on the island. The shadow of the mountain is also one of the sights of Etna, a dark-violet, triangular shadow (Baedeker) being thrown at sunrise over the surface of West Sicily, that is, on the land. I saw the shadow but imperfectly outlined, as the weather was not favourable at the time of my ascent. When at Nicolosi, on the south slope of Etna, I noticed at sunset a faint shadow of the mountain thrown against the eastern sky. I gathered from a short conversation with Prof. Ricco, the director of the Catania Observatory, when I told him of the shadow of the Hawaiian mountain, that the interest lay in its projection against the sky. It is doubtless akin to the spectre of the Brocken and other mountain spectres.

Some Previous Meteorological Observations on Mauna Loa.—.... Mr. Douglas, the botanist, who was subsequently found dead in a cattle-pit on Mauna Kea, spent a day on the summit of Mauna Loa in the middle of January, 1834. He mentions that a little way below the top the thermometer fell at night to 19° F. The wind on the top was N.W. The air at 11.20 a.m. was 33°, the hygrometer registering 0·5. He remarks that the great dryness of the air was evident without the assistance of the hygrometer (Hawaiian Spectator, vols. I and II, 1838-9).

Commodore Wilkes, in vol. IV of his Narrative of the United States Exploring Expedition, gives the following observations on the temperature and winds on the top of Mauna Loa between Dec. 23, 1840, and Jan. 13, 1841. Those on the temperature are incomplete, but they give a fair idea of the prevailing conditions. The degrees are in Fahrenheit’s scale.

The usual variation of temperature in the twenty-four hours is given as 17°-50°. The south-west was evidently regarded as the prevailing wind, and the clouds are spoken of as sometimes moving from opposite directions towards the same centre.