earth, many a bold aeronaut has gone above the clouds. Only after they have reached the height of 6,000 metres do they usually experience symptoms resembling those of mountain-sickness.
But on land these symptoms make their appearance at elevations far lower, and differing according to locality. In the Alps, definite symptoms first appear at 3,000 metres; in the Bolivian and Peruvian Andes, at 4,000 metres; higher still, in the equatorial Cordillera and on the Himalaya. In general, the elevation at which they first appear depends upon that of the line of everlasting snow, the lower limit of mountain-sickness being situated a little above the snow-line. The influence of the temperature is very evident. As for anomalies special to circumscribed localities or to individuals, the consideration of them would take us beyond the bounds we have set for ourselves here.
These grave and curious symptoms have been explained in many different ways by travelers, physicians, and experimenters. As for the native mountaineers, they solve the problem of their origin by referring them either to supernatural intervention or to the influence of noxious efliuvia. In the Ancles these efiluvia are reputed to be of an antimonial nature; in the Himalaya the cause is supposed to be vegetal poisons given forth from flowers, mosses, etc. These hypotheses need not detain us.
Among the many theories more or less tenable a priori, but none of which will stand the test of experiment, there is one which is almost universally accepted, and which reckons De Saussure among its distinguished supporters. It is known that the atmospheric pressure on a square centimetre of surface is 1.03 kilogramme. If we multiply this by the number of square centimetres of surface in a man's body, the product is something enormous. Take an average case, a pressure of say 15,000 kilogrammes. We are in equilibrium with this great pressure, they say; lessen the pressure, and the result is like the application of an immense cupping-glass over the entire surface of the body. The heart's action is now no longer sufficiently counterbalanced, and hence congestion and hseraoi-rhage of the mucous membranes and of the skin, engorgement of the blood-vessels of the face, cerebral troubles, and the rest.
It is amazing to find a theory so plainly at variance with elementary physical laws accepted by eminent men. What would be the result if we had to bear upon the surface of the body a pressure of 15,000 kilogrammes, and if every variation of the barometer added or subtracted from this sum one or two hundred kilogrammes?
Another theory, first offered by De Saussure, is far more worthy of attention. "On the top of Mont Blanc (4,810 metres)," says he, "the air is nearly one-half less heavy than at the sea-level; hence it results that if, in a given time, we pass through our lungs a given volume of air, that volume will represent only about one-half the weight of the same volume of the air to which we are accustomed. Hence there
