Sciences of Meteorology and Terrestrial Magnetism. 169 



grain will scarcely ripen, except in sheltered spots, beyond latitude 50° ; 

 here the cold winds are from the N.W., and trees cease before latitude 

 60 is reached. In Asia the warm and moist currents brought up from 

 the south over Central Asia by the floor or basis being extremely 

 heated in summer, " cause an arboreal vegetation to flourish up to 

 latitude 72° over ground perpetually frozen at the depth of a few feet." 

 Here the cold winds are from the N.E. 



(10.) It has been long the practice among physicists to assume, 

 on the ground of loosely collected records of mountain ascents, that 

 there is a direct simple ratio between height and temperature in the 

 atmosphere as we ascend. Eccent observations seem to show that all 

 these are incorrect, but no simple ratio has yet been grasped. It would 

 appear that Laplace's law, though not exact, gives a less error when 

 cempai'ed with balloon ascents than any other, — namely, that there is 

 a uniform fall of temperature when we ascend through heights increas- 

 ing in an ai'ithmetical series. The balloon ascents in 1852 at Kew, 

 under Mr. Welsh's care, to heights of 19,510 feet, 19,100 feet, 12,640 

 feet, and 22,930 feet, were conducted with the greatest nicety as to 

 instrumental contrivances. The temperatures were carefully observed 

 in correspondence with the heights of the quicksilver in the barometer ; 

 but it was from these barometric indications that the height was deter- 

 mined, according to Laplace's formula, in which his hypothesis of a 

 decrease in temperature is an element. Hence these temperatures, as 

 observed by Mr. Welsh, are exposed to doubt ; because, if Laplace's law 

 is incorrect, the heights are incorrect. The perfection of observation of 

 course would be that the heights of the balloons should be determined 

 by trigonometry, not by the barometer ; then we should have the 

 observed temperatures freed from every source of error, and perhaps 

 some new exact law of decrease might be deduced. It is not easy to 

 see, however, in what way this is to be attained. In the balloon ascents 

 of Mr. Welsh, strata of cloud were frequently passed through ; the 

 temperature, before decreasing, then rose ; after passing some distance 

 above the clouds the decrease was resumed, but the 7-ate of deci'ease was 

 greater below the stratum than above it. The disturbing causes of this 

 character being allowed for, Mr. Welsli considers that the observations 

 countenance no other hypothesis of decrease than that " this decre- 

 ment is uniform with the height." The average of all the ascents gives 

 a rate of 1° P. for 386 feet. The fall in ascending along the surface, as 

 up a mountain side, is much more rapid (see Article 14). In some of 

 the ascents, clouds, in the foi-m of a cirrous haze, were seen far above 

 the highest points reached. Half of the whole atmosphere is passed 

 through when the altitude of 18,500 feet is reached ; and air collected 



Vol. IV.— No. 1. z 



