MEDICAL HISTORY
OF THE
EXPEDITION TO THE RIVER NIGER
DURING THE YEARS 1841-2.

CHAPTER III.
Section III.

Meteorology.

The following tables comprise the monthly means of the daily meteorological observations that were made on board the Albert, as nearly as circumstances allowed, according to the ample register-forms provided by the Royal Society.

Owing to the great risk from exposure at night, the three a.m. observations were not taken regularly after the 19th August, 1841, when the vessels commenced the ascent of the Niger: therefore, from that time the barometric column contains only the mean results of the two maxima periods of nine a.m. and p.m., and the minimum period of three p.m.

The dew point as obtained by the method of Daniell; and the difference between the dry and wet bulb thermometers was registered at the same periods.

As some of my readers may not be fully acquainted with the principle upon which hygrometers act, I may here be allowed briefly to describe those that were used in the expedition.

Daniell's hygrometer. When air is saturated with vapour, and a colder body is brought in contact with it, a deposition of moisture immediately takes place.

The Florentine academicians first endeavoured to ascertain the hygrometric state of the air on this principle. It was afterwards adopted in a much improved form by Le Roi, and its application has of late years been brought to perfection by Professor Daniell.

The instrument of Daniell consists of two glass bulbs, free from air, and connected by a tube bent twice at right angles. The bulb which is at the end of the most depending limb is blackened, is filled three fourths with sulphuric ether, and has a delicate thermometer inserted within it, the scale of which projects a considerable way up the tube. The rest of the tube, and the other bulb (which is covered with gauze or fine muslin,) are filled with ethereal vapour.

When ether is dropped upon the muslin, the inclosed vapour is condensed, evaporation takes place rapidly from the ether in the blackened bulb, and cold is generated. Now, if there be much vapour in the air, a moist ring will at once appear on the blackened bulb; but if the air be comparatively dry, then the temperature of the bulb must be reduced by the addition of ether until the film of moisture is deposited.

The degree of temperature of the interior thermometer is to be noted the instant the moist ring is formed. This is called the dew point at the temperature of the surrounding atmosphere, which is indicated by a thermometer placed in front of the stand of the instrument.

Wet-bulb hygrometer. The rapidity of evaporation as a measure of the dryness of the atmosphere was the invention of Dr. Hutton, of Edinburgh, and the principle upon which Sir John Leslie’s hygrometer was constructed.

Those supplied to the expedition (Mason's) consisted of two delicate thermometers, placed upon a graduated scale of ivory with two arms, supported by a stand of brass upon a wooden rest. The bulb of one of the thermometers was covered with floss silk, which was kept moist by communicating with a water-holder, with which each instrument was provided. The amount of cold consequent upon the evaporation was indicated by the fall of the mercury in the tube of the moist-bulb thermometer: while the thermometer on the other arm of the scale showed the temperature of the ambient air. The difference of temperature between the two thermometers was noted, and a comparative estimate of the hygrometric condition of the atmosphere was formed; upon this principle "a thermometer having a thin film of water surrounding it will take a temperature depending on the following circumstances: — the air in contact (whether it move quickly or slowly) gives to the film of water, which is converted into vapour sufficient in quantity to saturate the space which the air occupies, just enough of heat to vaporize that water, and the reduction of temperature will be accordingly. Thus if the air (or space) be very dry, it will take up much vapour, but that vapour must have combined with much heat, in order to change its state from water, and the temperature of the air in the (now) saturated space is lowered accordingly."*

Comparisons with the Standard Barometer of the Royal Society.

At Plymouth. Passage to Madeira, and at Madeira: Passage to Teneriffe, and to the Island of St. Vincent in the Cape de Verds group. May 12th to 31st, 1841.

Island of Saint Vincent, in the Cape de Verds, until the 16th. Passage to Sierra Leone, and at Sierra Leone to the end. June, 1841.

Passage from Sierra Leone to Monrovia in Liberia; to the river Sinoe. At Sinoe, Cape Coast Castle and Accra, West Coast of Africa.

Observations taken off the Niger, and within that river.

Entered the Nun branch of the Niger August 13th. Steamed upwards on the 19th: reached Aboh on the 26th; Iddah, on the 2d September; the Confluence of Niger andTchadda, on the 12th September, remained there until the 21st: then proceeded to Egga, where we arrived on tbe 28th; steamed downwards on 4th October, and finally left the river on the 16th of that month.

At Fernando Po from October J7th to November 23d. Observations made by Mr. Rosher on shore. Sailed on the 23d to Prince’s island, St. Thomas’s island, and Rollas in the Bight of Biafra, and returned to Fernando Po December the 3d, 1841. Sailed again on the 18th for Ascension by way of Prince’s island, St. Thomas, and Rollas.

St. Thomas’s Isle, Anno Bon Isle, Passage to Ascension, and at Ascension. Jan. 1842.

ASCENSION.

Anchored on the north-west side of the island of Ascension: Observations made on board the ship.

AT THE ISLAND OF ASCENSION.

Barometrical, Thermometrical, and Hygrometrical Observations, made on board and at the Hospital of Ascension, during the above period.

The Means of the Monthly Averages.

The observations taken at the mountain are not included in the calculation of the averages.

Climate of Ascension. Ascension lies in latitude 7°.55.56 south, and longitude 14°.23.50 west, according to the chart of Lieut. Campbell, R.N. The island is about eight miles long and six broad, and is of the form of an isosceles triangle, the base extending from north and by east to south and by west, from which the sides run, the one south-east and easterly, and the other in an easterly and northeasterly direction.

The island is wholly volcanic. The green mountain, which is situated on the south-east side of the island, rises to the height of 2870 feet* above the level of the sea, the soil of the high lands round the peak is fertile, and is, to a considerable extent, under cultivation.

Vegetable life almost entirely disappears in the plains below, which present a scene of the most arid desolation, varied only by the unequal altitudes and changes of colour (from dark gray to deep red) of the old volcanic cones which are dispersed over them.

Although the seasons at Ascension cannot be distinguished by the usual tropical terms wet and dry, yet the warm period may be reckoned from the middle of December to about the same time in May; the rest of the year is comparatively cool.

The heat at the town quarter, which is on the north-west or lee side of the island, would be oppressive during the summer months, were it not that the atmosphere is renewed by the south-east trade which constantly blows over the island.

My own observation assigns the highest temperature to the months of March, April, and May; gradually falling until the middle or end of December, when it again begins to rise. The rapid advance in the growth of vegetation in the high lands surrounding the peak no doubt has tended to attract the clouds and vapours, and, consequently, to increase the precipitation of moisture in that district; but, independently of this cause, rains are, by all accounts, of much more frequent occurrence than they were ten or twelve years since, and add much to the coolness of the air by their quick evaporation from the porous surface of the cindery soil, as well as by the coldness due to the rain-drop itself.

The marines, of whom there are not less than between eighty or ninety,* are employed in quarrying, building, and other labours, exposed to the sun about eight hours daily, with the exception of Saturday; they are sometimes for months together without any other fresh provisions than an allowance of turtle, which is issued once a week, yet there is seldom one of these men to be found in the hospital: a fact affording strong evidence of the salubrity of the climate, and of the excellence of the discipline maintained on the island.

Means of Meteorological Observations, made by Lieut. Webb in the river Niger, from 2d to 27th July, 1842.

The weather was in general fine, although squalls from the s.w., with thunder and lightning, followed by heavy rains, were not unfrequent. The bed of the river presented a plain of sand, through which the river (now reduced to comparatively narrow dimensions) pursued a winding course. Snags (dead trees) abounded in the river: the stems of the trees and the bushes on the banks were covered with mud.

The following Observations made at Rabba, Lebba or Layaba, and Adda Kuddu, were kindly given me by Mr. William King, the intelligent Surgeon of the Ethiope, Mr. Jamieson’s steam-vessel.