he deposits of black pigmentary matter in the coloured cells of the epidermis or true skin; in consequence of which the Jollof and Mandingo inhabitants of these regions, but especially the former, are of a jet or lamp-black colour; whilst the Sarra Wollies and Footah Foolahs, who years ago migrated into these countries, become darker every generation, and the present inhabitants are two shades darker than the Moors of the interior, from whose stock they are descendants. It is common to characterise a very dark individual as "Jollof black," since the Jollofs, as a nation, present a more perfect black tint in their pigment cells than any other nation, I think, on the globe, at least that I have ever met with, which seems to prove most convincingly that to the temperature, exclusive of any other condition, we have to trace the colour of the skin.

I must state the fact, that the offspring of inhabitants of other parts of the coast, after residing for some time in this region, become as jet black in successive generations as any Jollof, although they themselves present no change, or very little, in the complexion. The converse of this I have noticed to be equally correct; for Jollofs who remain for a long time in other parts of Africa of a more equable and limited temperature, all things being equal, have offspring of a lighter complexion. Jollofs, in whatever favourable condition they may be as regards food, habitation, attention to bodily cleanliness when resident in their own country, retain that colour of their skin for which they are remarkable, and transmit the same to their offspring.

The heat also interferes with the general nutrition and vital operations of the body, either by disturbing the chylopoietic assimilation, or by acting indirectly through the nerve centres ; but one thing is certain, that the inhabitants are much more attenuated than in any other parts, and although generally tall, their muscles are ill developed, and they are incapable of severe muscular exertion, however short; their virile powers are quickly exhausted, and fecundity in the females is very limited. The Mandingo, especially, leads a sort of migratory life.

Even amongst some of the lower animals, which are expected

Effects of heat on respiration.

to be very hardy in their habits, such as goats and pigs, virility is very much interrupted. I observed, in many instances, that a goat, which, as a general rule, brings forth two or three kids, if impregnated during the hot weather and exposed to the full effect of the heat for two or three months, gives only one kid; evidently showing that the heat interferes with the full results of fecundation.

Through the heat the eliminating organs of the body are called upon to perform more work, while the formative organs are retarded in their elaboration. The kidneys during the hot weather are much more loaded with solid matter, much lithates and uric acid, showing increased degeneration and destruction of tissues; the sudorific glands increase in activity, and the perspiration contains an increased proportion of chlorides and other salts; whilst, on the other hand, we find a decrease in performance of the functions of respiration, digestion, and sanguinification. The operation of these causes tends to diminish the "span of life;" and the average of threescore years and ten is seldom attained by the inhabitants occupying these regions.

Besides being short lived, the inhabitants in this hot region grow early old; they appear in most cases about ten years older than they really are. A Mandingo or Jollof sixty years of age will have a long white beard, with grey hairs, and will give the appearance of a man of seventy years or more, which is not the case in other parts of the coast; and the natives themselves attribute it to the effects of the heat.

High temperature produces a very marked effect on the respiration of individuals residing in tropical climates in North Western Africa, where the temperature ranges in some parts of the year from 86° to 100°. The air is so rarefied in the afternoon that there exists an evident deficiency of oxygen for breathing purposes; an individual will be actually for hours in great distress, and pant for breath; his respiration becomes quicker, but he feels that he cannot take at a time sufficient air to relieve him; he finds immediate relief if he allows the heated

air to pass through a damp atmosphere before it enters into his lungs, and this he does by keeping his apartment continually wet by sprays or jets of water. Besides the want of a sufficient supply of oxygen in the air, individuals in these climates suffer also from a diminished purification of the blood, through a diminution in the exhalation of carbonic acid; the higher the temperature the less quantity of carbonic acid is exhaled.

It has been shown that the climate of the Gambia, Senegal, and the Casamanza, is the hottest in Western Africa; that the temperature during the hottest month in the shade is sometimes registered at 110°; also that the temperature of Sierra Leone, Liberia, the Gold Coast, and the Bights, are comparatively limited, and that the temperature during the hottest month rarely exceeds 90°. It follows that the inhabitants of the latter places exhale more carbonic acid than those of the former, and it is computed to be a little more than two-thirds. According to Vierordt,* a difference of 10° F., in man, produces a variation of rather more than two cubic inches of carbonic acid hourly exhaled.

I have previously stated that the inhabitants of this hot region are tall and attenuated; that their muscular system is ill developed. This has another effect, in diminishing the quantity of carbonic acid exhaled; for the more robust and muscular an individual is, all things being equal, the more carbonic acid he exhales-the height, weight, and capacity of the chest, &c., having very little to do with it. But these inhabitants, especially the Mandingoes, make up for this decrease by bodily exercise; they are a clever, athletic race, and are continually on the move, and consequently are always thus increasing the amount of carbonic acid exhaled; and when the exercise is moderate, it has been calculated to increase to onethird of the quantity exhaled during rest, which continues for some time after the cessation of the exercise.

The importance of these observations will be noticed, when we consider that the number of respirations per minute is from

* Physiologie des Athmens.

Atmospheric tides.

Atmospheric pressure.

Effects of trees.

15 to 26, or an average of 20; that at each respiration we take in 1.62 cubic inch of oxygen, and consequently make 28,800 cube respirations per diem, and take in and consume 46,656 cubic inches, or 26 cubic feet, or nearly a cubic yard of oxygen during that time. The great heat lessens this quantity to a very prejudicial degree. At each respiration a healthy individual takes in 20 cubic inches of air, and inspires 400 cubic inches in a minute, or 24,000 in an hour; such expired air contains about 4 per cent. of carbonic acid, and consequently 16 cubic inches per minute, or 960 per hour, and 23,040 in the 24 hours, viz., 3.529 grains of carbon. A diminished excretion of this substance vitiates the blood, and produces a very injurious effect on the system.

Every meteorologist in tropical climates must observe that there are tides in the atmosphere at different times in the twenty-four hours, occasioned by the presence or absence of the sun. According to Humboldt, the flow tide in the tropics commences at 9 or 9.15 A.M., and at 10 or 10.45 P.M.; and the ebb at 4 or 4.15 P.M., and at 4 A.M. This statement is pretty correct in Western Africa, except that the late ebb commences about 3 or 3.15 P.M.; the ebb is produced by the expansion of the atmosphere during the hottest part of the day, and the flow to the pressure of the mass of cold air.

The pressure of the atmosphere on a given body is directly as its density, or as its solid parts, to its own reacting interstices. In the mean human body the atmospheric pressure is reckoned at about 15 tons, or a total pressure of 32,400 lbs. ; but during the hot weather, or at an elevated spot in the tropics, when the atmosphere is much rarified, the amount is slightly diminished; for every degree of heat the air expands about T th part.

It is unfortunate that in Western Africa the local Government has in a great measure overlooked the beneficial effects that will be produced by taking advantage of the delightful properties of trees, in exhaling a certain quantity of aqueous vapour for the purpose of cooling the atmosphere: a more general

planting of umbrageous trees in the highways of the towns will operate to moderate the effects of atmospheric heat. An effort in this direction has been made by the planting of a large number of trees in the front street at Bathurst, in Government Yard at Sierra Leone, in Salt Pond Road, and Jackson Street at Cape Coast Castle; but there should be a more general one, and under a well-directed plan.

heat.

It is now the belief of every scientific man, that the further Internal we descend below the surface of the earth the greater is the temperature; and at 45 or 60 feet the increase becomes very perceptible; or in Europe, between the parallels of 48° and 52°, it forms the stratum of invariable temperature. At the depth of 100 feet the internal heat is equal to 2°.25 F., or 1° of R. of increase; but in intertropical Africa the stratum of invariable temperature is 1 foot below the surface, which cannot be due to solar influence. Since, says Professor A. Taylor :

"1. The existence of a zone of invariable temperature shows that the power of the sun must be limited to that portion of the earth's crust which is above it.

"2. If the interior heat of the mass of the earth were due to external or solar influence alone, the thermometer should fall just in proportion as we descend below the invariable stratum."

The reverse of this is, however, the fact; and hence, we must believe that below this invariable stratum we are arriving at a source of heat situated in the centre of the globe. Professor Alfred Taylor, after an elaborate investigation of this subject, arrived at the following conclusions:

1. That the temperature of the surface of our globe, including that of the air, earth, and sea, depends exclusively upon the quantity of heat transmitted to it from the sun, the heat thus received being again lost, partly by radiation into space, and partly by conduction downwards through the superficial strata.

2. That the chief loss takes place undoubtedly by radiation, and that it is by the amount of this last we learn the temperature of the medium (space) in which our globe is floating.