204 NINTH ANNUAL REPORT OF 



Oa no question has there been more discussion among meteorologists 

 than that in regard to the cause of the fluctuations in the height of the 

 barometer, especiafly during storms. John Dalton made the following 

 remarks on this subject more than sixty years ago ; and they are well 

 worthy of consideration in the present day, particularly since we possess 

 better opportunities of ascertaining their truth: 



1. "Tne barometer," says Dalton, "has little variation within the 

 tropics; while within the northern temperate zone, and doubtless within 

 the southern also, its range increases in going from the equator. The 

 mean annual range at Paris, for twenty years, was 1^ inches; the 

 greatest range, or difference between the highest and lowest observa- 

 tions, for the same term, was 2 inches. At Kendal, the mean range 

 for five years was 2.13 inches, the greatest range was 2.65 inches. 

 In Sweden and Russia the range is still greater. 



2. "In the temperate zones the range and fluctuations of the baro- 

 meter are always greater in winter than in summer. 



3. " The rise and fall of the barometer are not local, or confined to 

 a small district of country, but extend over a considerable part of the 

 globe, a space of two or three thousand miles in circuit, at least. 



4. "It appears that the mean state of the barometer is rather lower 

 than higher in winter than in summer, though a stratum of air on the 

 earth's surface always weighs more in the former season than the latter. 



5. " F. Laval made observations for ten days together upon the top 

 of St. Pilen, a mountain near Marseilles, which is 960 yards high, 

 and found that when the barometer varied 2| lines at Marseilles it 

 varied If inches upon St. Pilen. Now had it been a law that the whole 

 atmosphere rises and falls with the barometer, the fluctuations in any 

 elevated barometer would be to those of another barometer below it 

 nearly as the absolute heights of the mercurial column in each, which 

 in these instances were far from being so. Hence, then, it may be in- 

 ferred that the fluctuations of the barometer are occasioned chiefly by 

 a variation in the density of the lower regions of the air, and not by an 

 alternate elevation and depression of the whole superincumbent atmo- 

 sphere." 



I am quite aware that some of the deductions of Dalton require 

 to be a little modified. His first proposition, however, that the 

 range of the barometer increases in going from the equator towards the 

 poles, is amply borne out as a general law by observations in this coun- 

 try as well as in Europe. 



His fifth proposition is also correct, viz : that the fluctuations of the 

 barometer are occasioned chiefly by a variation in the density of the 

 lower regions of the air, and not by changes in the whole superincum- 

 bent atmosphere. To illustrate this, I will state a hypothetical case. 

 In the balloon ascent made by Mr. Walsh, alluded to in a previous lec- 

 ture, it was found that the barometer stood at 15 inches at the height 

 of 18,370 feet. If the whole air below this elevation had been occu- 

 pied by hydrogen gas, which is much lighter than common air, being 

 only about one fifteenth of its weight at the same pressure, this lighter 

 gas, possessing as much elasticity as common air, would be able to 

 bear up the upper stratum of the atmosphere, weighing 15 inches of 

 mercury ; but at the surface of the earth the barometer would fall 



