LIGHT SCIENCE FOR LEISURE HOURS. 



To speak of the confined air of the North Atlantic 

 Ocean is surely unreasonable. An ocean 2,000 miles 

 across, swept by more frequent storms than are experi- 

 enced in any other part of the globe, cannot be very 

 aptly compared to ' the bottom of a mine.' An inelastic 

 fluid flowing steadily over a rugged surface shows no 

 trace, or but the slightest trace, of the nature of that 

 surface, by any variations of its own level. But it is 

 still less conceivable that an elastic fluid should be in- 

 fluenced in the manner suggested. In fact, if this 

 happened, we should no longer be enabled to determine 

 the heights of mountains by barometric observations ; 

 for according to the theory the air should extend to a 

 greater height above mountains than above plains ; and 

 as regards comparison between a barometer at the foot 

 of a mountain and one at the summit, we might argue 

 that the barometer in the valley, compared with a 

 barometer at the same level in a plane district, ' is 

 pressed upon, not only by the air clear of the mountain 

 tops, but also by the air confined within the valley,' so 

 that the altitude over the valley is greater by some 

 hundreds of feet than the altitude over a plain at the 

 same level as the valley ; and thus, before we could 

 determine the height of the mountain above the level 

 of the plain, we should have to determine the exact 

 effects due to the confinement of the air in the valley. 

 We know that, on the contrary, the average barometric 

 pressure in the most confined valley does not differ 

 appreciably from the average pressure over the most 

 widely extended plain at the same level. 



