68.5 63.5 650 63.6 66.7 67.7 67.6

...

Feb. 62.7 63.3 68.7 62.6 62.9
March, 57 56.2 54-7 59.8 58.1 56.2 56.3 53.7 54 53.6 55.5 55 3 52.8 52.2 50-8
April, 53.5 58.4 53.5 62.6 65.2 63.6 64.6 64.2 61.7 65.7 71.3 68.8 63.1 58-2
May, 61.5 60.5 60-8 60.5 58.7 63.1 60.1 61 59.4 62.7 61.1 64.3 61.3 64.5 62.2
58.5 59.5 67 71.4 67.5 66.1 74.5 65-7 63.1 64.1 62.8 61.3 61.7

June, 59

...

161

CHAPTER VIII.

BAROMETRIC PRESSURE-OZONOMETER-AMOUNT OF CLOUD

NOTATION OF THE WEATHER.

principle of.

IN 1646, through the independent researches of Pascal and Barometer, Torricelli, it was ascertained that a column of water 34 feet high, and a column of mercury 30 inches high, each exactly counterpoises a column of air of an equal base, extending from the level of the sea to the summit of the atmosphere; and the next year the former observer discovered that there exists a distinct and constant relation between the rise and fall of the mercury in the Torricelli instrument and the changes in the temperature of the atmosphere, produced by rarefaction or condensation, by elevation or descent, each of which is dependent on the unequal distribution of heat.

maximum

My observations on the barometric pressure in Western Period of Africa are, unfortunately, very limited, but it appears that the and minimum maximum height of the barometer takes place during the observations. blowing of the harmattan winds, and consequently from December to February; and the minimum during the tornado periods, at the beginning and after the rains. The barometer always falls at the approach of rain or wind, and begins to rise during it. The cold, dry, dense air of the harmattan raises the barometric column, whilst the rarefaction of the air preceding the tornado storm depresses it. The greatest range of the barometer in the Gambia, Senegal, and Casamanza, occurs in the month of January; in Sierra Leone in April or May; on

L

measuring

mountains.

the Gold Coast and Lagos in September; the least range in the first named places, in March, April, and May; in the second and third places, in February.

It has been ascertained that for every inch which the mercury in the barometer rises or falls, the boiling point of water is increased or diminished by 1°.76; therefore, if the barometer rises or falls one-tenth of an inch, there is an elevation or depression of the boiling point by 0°176 F. Mr Fahrenheit took advantage of this to ascertain the height of Barometer in mountains, and "he proposed, that in order to find the difference in feet between the upper and lower stations, pure water should be boiled in an open vessel at both stations, and the difference of temperature at which it boils should be multiplied by 5.30, which will give a close approximation to the height of the upper above the lower station. From the conjoined temperatures of the atmosphere at the stations, 64° are to be subtracted, and the remainder is to be multiplied by the one-thousandth part of the height already found, to which it is to be added. This is the correction for the difference of the temperature at the stations. A further correction is necessary for the figure of the earth and the latitude of the place."* This, on the West Coast of Africa, amounts to about one foot in addition to every thousand of the calculated elevation.

As an example, let us imagine water to boil at the level of the sea opposite Freetown at 212° F., and at a height by the side of Sugar-Loaf Mountain at 210°. The temperature at the sea level being 84°, and that at the higher station 80°, the actual height is thus found,

=

Temperature of water, 212° 210° = 2 x 530 = 1060 approximate height.

Temperature of atmosphere, 84° + 80° = 164 - 64 = 100 × 1.060 100 + 1060 2060.

=

Latitude, 2.060 x 12.060 2060 2062-06. The actual height of the spot examined.

Lately a sensitive and very accurate barometer was invented

*Pickford's Hygiene, p. 13.

barometer,"

and uses of.

by M. Vidi, a native of France, which is called the "Aneroid "Aneroid Barometer."* It "consists of a flat circular metallic box, 4 description inches in diameter, and 1 inch thick." Within is an exhausted copper drum, having a thin corrugated upper plate or head. From the centre of this last an upright metallic shaft springs, "which, as it rises or falls by the varying amount of atmospheric pressure exercised on the drumhead," multiplies, through ingenious and complicated springs and levers attached to its upper end, "the delicate movements thus propagated to it;" these are marked by an index which traverses a graduated dial, corresponding in inches and tenths with the scale of the mercurial barometer. I have given a few observations taken with this instrument in the Military Hospital at Sierra Leone.

A column of air, extending from the surface of the sea to the top of the atmosphere, which is equal to about 45 to 50 miles, exercises a pressure upon one inch of surface, of 15 lbs., or more correctly, 14.75 lbs.; and according to Professor Schmid of Jena, the entire weight of the atmosphere on the whole surface of the earth is 612,489,851,187,053 tons. An average-sized man exhibits a surface of 15 square feet, or 2160 square inches, and therefore at the level of the sea sustains a total atmospheric pressure, equal in all directions, of 32,400 lbs., or nearly 141 tons.

The barometer presents steady diurnal changes, depending on the amount of aqueous vapours. There are two maxima, viz., one at about 9 A.M., and the other at about 10 P.M.; and two minima, one between 3 and 4 P.M., and the other at about 4 A.M..

For the rules necessary to be observed in the use of the barometer as a weather glass, see the introductory chapter.

Western

At present all the stations of the British, Dutch, and ozone in Spanish Governments on the West Coast of Africa are more or Africa. less confined to the sea coast. The French Government has most of its stations on the sea coast, and a few in the interior. These stations are well supplied with ozone from their posi**Aviv, without, and powdns. fluid.

Clouds, uses

and causes of their colour.

tions, ozone being abundantly generated from the surface of
the sea; it is the most powerful oxidising agent yet known,
and it has a powerful affinity for malarious or paludal poisons,
which it rapidly decomposes; it is more plentiful at night,
when malaria is very rife, than in the day, when it is less so.

It is a natural ingredient of atmospheric air, and consists of
three equivalents or atoms of oxygen and one of hydrogen. Ac-
cording to Schönbein, it is one of the "chemical antipodes and
antidotes to all oxidable miasmatic and malarious gases and
emanations disengaged from putrefying animal and vegetable
substances, converting them into innocuous matter, and thus
purifying and sustaining the entire salubrity of the atmo-
sphere. In short, so hostile to organic miasmata, so incom-
patible with them is ozone, that the presence of the latter
enables us to affirm the absence of the former, and the healthi-
ness of the locality in which it is found." The observations in
Freetown show the existence of but small quantities of ozone.
This must be due to its destruction by miasmatic agencies
which it decomposed; and since vegetation and the foliage of
trees generates a large quantity, the Sierra Leone Government
should take advantage of this, and plant trees in the streets of
Freetown.

In another place I shall fully consider the effects of ozone in the atmosphere in the treatment of malarious fevers. There has been no ozonometric observations made in the Gambia, and I have not heard of any in Senegal.

I have in the introduction stated that clouds are visible collections of minute globules of water in suspension in the atmosphere. They are useful in moderating the temperature on the surface of the earth, preventing vegetation from being scorched by day, and radiation of the earth's heat at night, and consequently opposed to the deposit of dew. Through the clouds the evaporation which is continually taking place on the surface of oceans, seas, and rivers, is carried through the agency of the wind into the interior of continents in the form of rain. The beautiful cerulean colour of the sky, and the variegated

A