'80 ' PHILOSOPHICAL TRANSACTIONS. [anNO l66d, 



place, where there is a mercurial barometer, as well freed from air as mine, 

 if it appear to weigh precisely a dram, and the mercury in the baroscope there 

 stand at just 29^ inches, we may conclude the gravity of the atmosphere to 

 be sensibly equal in both those two places, though very distant. And though 

 there be no baroscope there, yet if there be an additional weight, as for in- 

 stance, the 1 6th part of a grain requisite to be added to the bubble, to bring the 

 scales to an equilibrium, it will appear that the air at this second place is, at that 

 time, so much heavier than the air of the former place was, when the mercury 

 stood at 294- inches. 



But in making such comparisons, we must consider the situations of the 

 several places ; for if one of them be in a vale or bottom, and the other on the 

 top or some elevated part of a hill, it is not to be expected that the atmosphere 

 in this latter place should gravitate as much as the atmosphere in the former, on 

 which a longer pillar of air insists. And this suggests a method of ascertaining 

 the absolute and comparative height of mountains, &c. by noticing, by the 

 baroscope, the difference in the weight of the air at the bottom and at the 

 top.* 



The Phases and Revolutions of the Planet Mars about his Axis. 

 By Mr, Hook. N' 14, p. 239. 



On the third of March, 1665-6, though the disposition of the air was not 

 good, yet I could see now and then the body of Mars appearing of the form A, 

 fig. 1, pi. 2, which I presently described by a scheme ; and about 10 minutes 

 after, it had through the glass the appearance as in the scheme B. This I was 

 sufficiently satisfied, by very often observing it through the tube, and changing 

 my eye into various positions, that so there might be no kind of fallacy in it, 

 could be nothing else, but some more dusky and spotted parts of the face of 

 this planet. 



March 10, finding the air very thick, I made use of a very shallow eye- 

 glass, as nothing appeared distinct with the greater charge ; and saw the ap- 

 pearance of the planet as in C, which I imagined might be the representation 

 of the former spots by a lesser charge. About 3 o'clock the same morning, 

 the air being very bad (though to appearance exceeding clear, and causing all 

 the stars to twinkle, and the minute stars to appear very thick) the body seemed 

 like D ; which I still supposed to be the representation of the same spots 

 through a more confused and glaring air. 



• Thus we find that the method of measuring heights by the barometer, is nearly as old as tlie 

 instrument itself. 



