away the carbonic acid to the vast forest tracts of the globe the grand ventilation of Nature; so that the atmosphere, as a whole, gets back, volume for volume, as oxygen, all the carbonic acid produced in denselycrowded cities.

The influence of forests on rainfall, which was already noticed in the days of Christopher Columbus, has been so often written about that I need say little on this subject. I have already alluded to the steam issuing from the woods on the mountains of Waldeck after heavy rain in summer, a phenomenon showing that the atmosphere above the woods is cooler than the air in their interior. I have also been struck by the rapid manner in which the evenings cool after hot days in these woody districts. The forests presenting a rough surface radiate heat more rapidly than the smooth surface of the open country. Thus, after very hot days in June, when the thinnest cloth coat, or no coat at all, was most acceptable, I found it necessary to put on an overcoat some time before sunset.

This radiation, so marked from the rough surfaces of forest lands, will readily account for the condensation of more cloud, and consequently a greater rainfall in wooded districts; hence also increased fertility, and a more prosperous condition of the inhabitants.

A well-known French hygienist traces the arrest in the increase of population, and even depopulation, in thirty departments of France to the disappearance of forests in the mountainous districts.

CHAPTER XV.

Cold Air more dangerous than hot Air-The late Dr. Meisser's Opinion-Effects of Sojourn in the Tropics-Temperature Observations-Altitude, Dryness, Humidity-Carbonic Acid in London Fogs-Air over Stagnant Water-Hydrocarbon Gases in the Air of certain Localities-The Air of MinesAir of Bedrooms--Air of Fermentation-Air of Wells and Sewers-Air over active Volcanoes-Air of Mountains— Effects of Arsenic-Air of Treeless Plains-Air of the Arctic Regions, and that near the Snow-line on Mountain-slopes— Air of the Tropics-Air in Epidemics of Cholera.

Cold air produces a higher death-rate than hot air, as was professed in 1850 by the late Dr. Meisser, the distinguished Professor of Comparative Anatomy at the University of Brussels, and his opinion was fully confirmed by Dr. Benjamin Ward Richardson of London, in 1890, or forty years later.

Meisser especially pointed out that cold was more dangerous than heat to aged persons. The saturation of the air with moisture increases the unfavourable effects of a cold atmosphere. The danger from consumption is much greater in winter than in summer.

With regard to hot air, it is known, at present, that Europeans who have lived for some time in the tropics are much less able to withstand the heat than are new

comers. The Anglo-Indian statistics supply us with direct proof of the diminished vitality (or resisting powers) of Europeans living for a long time in the tropics.

The temperature of the air follows the course of the sun; it is coolest towards the break of day, just before sunrise, and hottest about two or three in the afternoon.

To obtain a forecast for the day, it is necessary to observe the thermometer always at the same hour, say at nine o'clock in the morning. Its indications will then serve to establish whether the day will be colder or warmer than the preceding day.

In very variable weather the thermometer will fluctuate considerably during the twenty-four hours, both by day and by night, as cold or warm currents of air affect it; but such sudden changes are not common. They are more frequently observed with us in England during the cold winter weather, when the night temperature is sometimes higher than the day temperature, just as with a patient suffering from fever.

The air on a cloudy night is always warmer than it is on a clear night when the Earth radiates its heat into space. Clouds check this radiation. We cover up plants to preserve them from cold on clear nights; their radiation often cools them down to freezing-point in spring, and in summer causes them to be covered with dew. When the sky is overcast there is less danger from frost, and no dew is deposited.

The temperature of the Earth's atmosphere is evidently affected by the position of the planets. The

G

approach of Mars to the Earth in 1892, when it appeared in the sky as brilliant as Jupiter, was followed in 1893 by most intense heat at Bombay, New York, London, etc., etc. (the law is that when bodies (or atoms) approach, heat is the result, and when they move away from each other cold results). The going away of Mars was probably the chief cause of the intense and prolonged winter of 1894-95, which will be long remembered in London (especially by the Water Companies), and in the South of France and northern Italy, where its effects were first felt.

The cold days of May (about the 10th to 12th) are most probably to be ascribed to the interposition at this period between the Sun and the Earth, of the November meteor streams (13th and 14th); to the melting of snow and ice on the mountains, by which enormous amounts of atmospheric heat are absorbed, and to the presence of floating icebergs in the Atlantic.

With regard to altitude in the atmosphere, it has recently been found in India that 7000 feet is about the best height for a permanent residence for Europeans, 5000 feet being the lowest level at which malarial fever can be avoided.

In South America yellow fever breaks out only in localities where the temperature reaches to 70° Fahr. and above. At elevations where this temperature of 70° is never exceeded, the inhabitants are almost exempt from this terrible epidemic.

Dryness of the atmosphere, with liability to dust, cold, fogs, winds, and rapid fluctuations of temperature, con