VOL. LX.] PHILOSOPHICAL TRANSACTIONS. 17 



reflection through that piirt of the planet's atmosphere where the contact was to 

 happen ; anrl tlie well-defined streaks of light following it, he takes to have been 

 the sun beams passing between mountains on the surface of Venus's globe. 



X. Of some Improvevients made in a New fVheel Barometer, Invented by Keane 

 Fitzgerald, Esq., F. R. S. p. 74. 



Mr. F. gave a former description of a wheel barometer of a new construction, 

 with registers to mark the rise and fall of the mercury, which were published in 

 the 52d vol. of the Philos. Trans, for the year 1 761. And he here otters fur- 

 ther improvements of the same, which are described. 



The exactness and facility with which an account of the variations in the 

 weight of the atmosphere may be kept, with the help of a barometer of this 

 kind, Mr. F. thinks must be very evident. He often found, by extraordinary 

 variations that have happened in the night, when the wind has risen considerably, 

 how little the observations made with common barometers are to be depended on; 

 and several times found bv the registers, that the mercury had sunk 50 or 60 

 divisions; and one night particularly had sunk ] 17 degrees, and returned within 

 a degree and half of the place he had marked it on going to bed. When a 

 strong gust of wind rises, one may very plainly perceive the index of this latter 

 barometer to sink several divisions, and rise again as it abates. Besides the satis- 

 faction that a barometer of this kind might afford to a curious observer, Mr. F. 

 imagines it might also be usefully applied to the finding the height of the atmos- 

 phere; with a much greater degree of exactness, at least, than can well be af- 

 forded by any other. It is generally allowed from experiments, that a column of 

 air 72 feet high is equal in weight to ] inch of water of the same base; so that, 

 if the air were of equal density throughout, the atmosphere could be little more 

 than 5 miles high. But as the density is found to decrease by the difference of 

 pressure, and the air to be more rarefied or expanded in proportion to its distance 

 from the earth, it seems reasonable to conclude, that if by accurate experiments, 

 the ratio of its decrease were found regular in proportion to the distance from the 

 earth, its height might be estimated with a much greater degree of precision 

 than it has been hitherto ; though it seems generally allowed that its real height 

 cannot possibly be ascertained. The impossibility of observing the difference of 

 the pressure of the atmosphere at small distances with accuracy by a common 

 barometer, the scale of which is but 3 inches, is very evident; how far this in- 

 strument, the scale of which is go inches, might be conducive to the purpose, 

 is submitted to the judgment of others. Mr. F. imagines it would not be diffi- 

 cult, with a proper teakle, to raise a barometer of this kind gently, as high as 

 200 feet; and if it were raised from the ground, and let down again from each 

 distance of 20 feet, the registers would mark very exactly to the 600 part of an 



VOL. XIII. D 



