them nearly but not quite the same. An increase in the amount of carbonic acid evolved at places increasing in altitude was attended with an actual decrease of the volume of air expired also reduced. This result again agrees with that obtained by Mr. Mermod. It applies in my case to greater altitudes, attended with differences of temperatures at the various stations; while, as already stated, the temperature of the air was the same at Mr. Mermod's two stations. As the amount of carbonic acid expired is more at the higher stations than at the lowest, and the volume of air breathed (reduced) less at the higher stations, it must follow that the proportion of carbonic acid expired will be greater at the higher than at the lowest station; we find accordingly that the mean for the high stations yields 4.9 per cent. of carbonic acid in the air expired, while the experiments at the lowest station give 4.1 per cent. From observations made with and without the face-piece, it follows that I breathed into the open air slower, and apparently deeper, through the mask than without it. This accounts for the small number of expirations I noted per minute in all my experiments. Relatively, however, the results obtained on this point may be accepted, as the breathing was carried on always in the same way while sitting and walking respectively. We find an increase in the frequency of respiration from an altitude of 8,115 feet to that of 13,685 feet, but hardly any such increase between the altitudes of 1,230 feet and 8,115 feet. This greater number of respirations per minute between the St. Bernard and Breithorn was not observed to progress proportionally with rising altitudes from 8,115 feet, as will be seen in the table, and there must have been some other cause besides increased elevation to account for it. Mr. Mermod found no difference in the frequency of respiration between 466 feet and 3,609 feet; and here, again, for low stations I agree with him; but I cannot follow him in concluding that, therefore, the phenomenon is not altered at greater altitudes. The volume of air reduced, exhaled per respiration, varied at the several stations, but followed no regular change relatively to altitude; it ranged from 510 cub. centims. at St. Theodule, to 760 cub. centims. at the St. Bernard. Experiments made while walking on Level Ground, or ascending. In every one of these experiments I walked for a short time, say two or three minutes, or longer, before commencing to collect the air expired. They are fewer in number than those made sitting, and less satisfactory, as it is impossible to depend upon the degree of muscular exertion being the same in comparative experiments while in the act of walking or climbing. The results have been disposed in a tabular form, and appear to show that while walking on level ground, when a certain altitude is reached, there is a decided fall in the amount of both the carbonic acid and air expired. Yvoire and the St. Bernard gave nearly the same results; but when the height of the St. Theodule Pass was attained (10,899 feet), there was a reduction in the expiration of carbonic acid while walking on level ground from 2.249 grms. at Yvoire, and 2-457 at the St. Bernard, to 1919 grms.; on the summit of the Breithorn there was a further fall of carbonic acid expired to 1.886 grms., while the volume of air expired per minute was reduced from 24.77 litres to 19.3 litres; but the experiments are not numerous enough to allow of any but very general results. The same remark applies to the experiments made walking up hill. They certainly show, however, that walking up rapidly over rocks and grass patches at or below the elevation of the St. Bernard, yield most carbonic acid, the amount being as much as 3.156 grms. per minute at the St. Bernard, which was attended with the inhalation of the largest volume of air breathed. Ascending quickly at the height of St. Theodule caused a considerable elimination of carbonic acid through the lungs, amounting to 2.972 grms. On the other hand, walking leisurely up hill at the St. Bernard gave rise to the production of no more carbonic acid than quick walking on the level ground at that same station; indeed, the amount was a trifle less. These experiments, therefore, give an idea of the extreme quantities of carbonic acid expired at various altitudes under moderate and great muscular exertion, and appear to show that at great elevations, such as that of the summit of the Breithorn, and perhaps lower, the body is less able to take in a sufficient amount of air for the supply of carbonic * Appears rather high. acid necessary to long-continued exertion, which supply becomes thereby reduced in quantity. EXPLANATION OF THE FIGURE. A. India-rubber bag acting as a diverticulum. B. Pipette delivering 100 c.c. of the barium solution. C. Tube for the analysis of the air expired, of a capacity of about 1.5 litre. If the relative humidity of the atmosphere should fall at increasing altitudes this might be considered as an additional cause of loss of heat the body must experience during Alpine ascensions. Professor Plantamour, of Geneva,* from observations made at Geneva and the Great St. Bernard, concludes that there is no marked difference between the hygrometric states at various altitudes. According, however, to Dr. Lombard, who has considerable knowledge and experience of climate, the air appears to be, as a rule, much drier above 1,500 metres than below that altitude. III. "On Stresses in Rarefied Gases arising from Inequalities of Temperature." By J. CLERK MAXWELL, F.R.S., Professor of Experimental Physics in the University of Cambridge. Received March 19, 1878. (Abstract.) 1. In this paper I have followed the method given in my paper "On the Dynamical Theory of Gases" (Phil. Trans., 1867, p. 49). I have shown that when inequalities of temperature exist in a gas, the pressure at a given point is not the same in all directions, and that the difference between the maximum and the minimum pressure at a point may be of considerable magnitude when the density of the gas is small enough, and when the inequalities of temperature are produced by small solid bodies at a higher or lower temperature than the vessel containing the gas. 2. The nature of this stress may be thus defined: let the distance from the given point, measured in a given direction, be denoted by h, and the absolute temperature by 0; then the space-variation of the temperature for a point moving along this line will be denoted by and the space-variation of this quantity along the same line by do dh' d20 dha ᏧᎾ There is in general a particular direction of the line h, for which is a maximum, another for which it is a minimum, and a third for dh' which it is a maximum-minimum. These three directions are at right angles to each other, and are the axes of principal stress at the given point; and the part of the stress arising from inequalities of temperature is in each of these principal axes a pressure equal to― 3 μ where u is the coefficient of viscosity, p the density, and the absolute temperature. 3. Now, for dry air at 15° C., μ=1.9 × 10-4 in centimetre-gramme second measure, 1 and 0-315, where p is the pressure, the unit · ρθ Ρ of pressure being one dyne per square centimetre, or nearly onemillionth part of an atmosphere. If a sphere of one centimetre in diameter is T degrees centigrade hotter than the air at a distance from it, then, when the flow of heat has become steady, the temperature at a distance of r centimetres will be Hence, at a distance of one centimetre from the centre of the sphere, the pressure in the direction of the radius arising from inequality of temperature will be Ρ 0.315 dynes per square centimetre. 4. In Mr. Crookes' experiments the pressure, p, was often so small that this stress would be capable, if it existed alone, of producing rapid motion in small masses. Indeed, if we were to consider only the normal part of the stress exerted on solid bodies immersed in the gas, most of the phenomena observed by Mr. Crookes could be readily explained. 5. Let us take the case of two small bodies symmetrical with respect to the axis joining their centres of figure. If both bodies are warmer than the air at a distance from them, then in any section perpendicular to the axis joining their centres, the point where it cuts this line will have the highest temperature, and there will be a flow of heat outwards from this axis in all directions. ᏧᎾ Hence will be positive for the axis, and it will be a line of maxidh mum pressure, so that the bodies will repel each other. If both bodies are colder than the air at a distance, everything will be reversed; the axis will be a line of minimum pressure, and the bodies will attract each other. If one body is hotter, and the other colder, than the air at a distance, the effect will be smaller; and it will depend on the relative sizes of the bodies, and on their exact temperatures, whether the action is attractive or repulsive. 6. If the bodies are two parallel disks, very near to each other, the central parts will produce very little effect, because between the disks the temperature varies uniformly and =0. Only near the edges will d'o there be any stress arising from inequality of temperature in the gas. 7. If the bodies are encircled by a ring having its axis in the line joining the bodies, then the repulsion between the two bodies, when |