A'croslutlon, 37I 



thermometer at 21 degrees, he launched two parachutes of 

 difterent sizes, and loaded with equal weights, in order to 

 calculate the resistance of the air. The second, which was 

 launched a hundred toiscs higher than the first, ieil with 

 much greater velocity, but did not unfold itself till it had 

 pa-?cd over a double space. At 51 minutes after twelve he 

 passed between two -large clouds, which seemed to open 

 to aftbrd a passage to the balloon. The form of these masses 

 of vapour is oblong. They resemble rags suspended above 

 the earth. Their upper parts do not form in their ag- 

 gregate a smooth surface, as appears to those who look at 

 them from the earth ; on the contrary, they resemble long 

 pyramids. This etfect ought to be ascribed to caloric, which, 

 if we may use the expression, converts these masses into 

 Montgolliers^ the elevation of which is proportioned to the 

 density of the atmosphere. They appeared to Mr. Robert- 

 son to plunsfc towards the earth, in consequence of an op- 

 tic effect resulting from the apparent immobility of the bal- 

 loon, which, however, was rising at the rate of 50 feet per 

 second. When the thermometer indicated one degree above 

 freezing, and the barometer stood at 1 5 inches, Mr. Robert- 

 son set at liberty two pigeons, which descended with th" ra- 

 pidity of lightning, without moving their wings, and in a plane 

 slightly inclined. When the barometer stood at 14 inches 

 he let off a third pigeon, which, having fluttered about for 

 a moment with dilficuUv, perched on the net-work, and 

 would not quit it. Two butterflies let go at the same time 

 tried to use their wings, but in vain, as the air was too rare; 

 they never quitted the car, and fluttered, but in a very feeble 

 manner. Tuider exposed to a convex glass of six inches focus 

 did not catch 'fire till the end of some minutes. The rays 

 refracted by the prism no longer exhibited lively and distiiict, 

 but weak and confused, colours. Weights attached to a 

 spring balance had lost one half of their gravity. The mag- 

 netic needle began again to put itself in motion. By means 

 of a very ingenious mstrument invented by M. Hez, me- 

 chanician, he inclosed four inches of the surrounding air 

 along with mercury, and marked exactly the point where 

 the air and the mercury v/ere united. When he reached the 

 earth, he found that the latter filled the whole tube within 

 a tenth. 'I'hi.s important experiment seems to prove, that 

 in the upper regions there exists nothing but vapours, and 

 no atmospheric air. If this conjecture of Mr. Robertson 

 be verified, there will be no reason why people may not 

 ascend to a much greater height than that of 3<j70 toiscs, to 

 whieh wc know some have ascended : but for this purpose 



A a 2 3L ballouu 



