510 MR OMOND ON THE 



from the computed differences of barometric pressure in Scotland as in Switzerland ; so 

 that what explains the one may be fairly looked on as at least a possible explanation of 

 the other. An examination of the table of temperatures on previous page shows at once 

 that the air between Santis and Zurich, whose weight is measured by the difference of their 

 respective barometers, has not a temperature anything like the mean of the Santis and 

 Zurich temperatures, but is considerably higher, as every station intermediate in height 

 gives a value higher than this mean. If we assume the mean temperature at 7 a.m. to 

 be 28*4 instead of 21 '1, and at 1 p.m. 30*7 instead of 215*1, we will find the differences 

 of barometric pressure computed from such temperatures to agree with the differences 

 observed between Santis and Zurich. This conclusion, that the intermediate air is 

 warmer than the average of the higher and lower station, is obvious and necessary from 

 the temperatures in the table on previous page, and is the same as the hypothesis put 

 forward to explain the Ben Nevis and Fort- William anomalies of barometric reading. 



Other cases of warm air on Santis and cold at the valley stations all point to the 

 disturbance of temperature being in the lowest strata of the air. In the three months 

 January, February, and March 1893, the temperature at 7 a.m. on Santis exceeded that 

 on Rigi only twice, and then only by 1°*8 and o, 9 respectively ; while Santis exceeded 

 Zurich six times, the amounts being 4°'8, 6 c 'l, 9°"7, 9 0, 6, 4°'9, and 1 0, 8 ; and this, 

 although Santis is only 2300 feet higher than Rigi, and is 6600 feet above Zurich. 

 In the same period, Rigi was on twenty occasions warmer than Zurich at 7 a.m., the 

 greatest being 18°'7 on January 9th. 



The cause of this peculiar arrangement of temperature in winter anti-cyclones must 

 be looked for in the character of the air brought over the district under such conditions. 

 A slowly descending mass of air settling downwards from the cold, rare regions of the 

 upper atmosphere, and flowing outwards from the area of highest pressure on the 

 earth's surface, is the generally received description of an anti-cyclone. This air starts 

 on its downward course with little water vapour and little dust in it ; it has no oppor- 

 tunity of gathering any till it reaches the surface of the ground, for all the vapour and 

 dust in the air ascend from below, and it is steadily heated by compression as it descends. 

 Such air is almost perfectly diathermanous ; it is a bad absorber and bad radiator of 

 heat ; therefore it does not lose the heat that is developed in it by compression during 

 descent, as it cannot get rid of it in the only way possible, by radiation into space. 

 This heating, when undisturbed, produces in dry air a rise of about 5° for every 1000 

 feet of descent, an amount considerably greater than the normal rate of change of 

 temperature with height. Hence the air comes down to the mountain summits 

 comparatively warm, and very dry ; as soon, however, as it brushes the surface of the 

 ground, or comes in contact with air that has been lying near the ground, it begins to 

 absorb moisture and gather dust ; the heat required to evaporate the water, which thus 

 saturates the lowest stratum of the air, will be largely taken from the air itself, which 

 is thus left cold and damp, as we observe at all low-lying stations in such weather. 

 This air, moreover, being full of dust and condensed water particles, is a good radiator, 



