4g6 PHILOSOPHICAL TRANSACTIONS. [aNNO 1720. 



of Mercury in my tables were assumed, the same right ascensions and declina- 

 tions of those stars would be deduced from him, as from Venus: and to my 

 great satisfaction, I found, on trial by an exact calculus, that I had the same 

 right ascensions now as before, in none of the 4 differing quite half a minute; 

 so that these stars may securely be added to the catalogue, and the appulse of 

 Mars to them be observed in very long telescopes, in October next, to the 

 further ascertaining the immense distance between the sun and earth. 



Hence it will also appear that our mercurial numbers are, at least at this 

 time, and in this part of his orb, not less exact than those of Venus. And 

 whereas this planet scarcely ever appears with us out of the sun's beams, and 

 always low, and therefore under great refraction ; this way of observing takes 

 off all the uncertainty thence accruing ; and when once the zodiac shall be 

 completed with the stars that are wanting to fill up the vacant places, it will be 

 easy at any time, by this method, to observe Mercury or a Comet within the 

 sun's beams, with the same certainty as if it were remote, and out of the 

 neighbourhood of the horizon, where the different vapours near the earth 

 render the appearances of the stars somewhat dubious, on account of the irre- 

 gular refractions. 



^ Proposal for measuring the Height oj Places^ by Mr. Patrick's Barometer, 

 in which the Scale is greatly enlarged. By the same. N° 366, p. 1 i 6. 



Since Torricelli first found that the Mercury in an inverted tube was in 

 aequilibrio with the whole column of air over it; and that the weight of the 

 incumbent column was various, according to the different dispositions of the 

 air, in respect of serene fair weather, and of rainy, windy, or otherwise tem- 

 pestuous weather; there have been several attempts and contrivances to make 

 its minute variations more sensible. And first the wheel barometer was 

 thought of; which certainly shows these variations with great exactness; but 

 it is only proper for a fixed station, and not easy to be removed ; which circum- 

 stance is necessary for the principal use this instrument is applicable to, and 

 which I would recommend it for. 



The next thought for this purpose, was that of Mr. Hubin, described in 

 Phil. Trans. N° 184, who returning the tube of the barometer, as an inverted 

 syphon, made a large dilatation in the ascending leg, where the Mercury 

 ascended, as its altitude in the other part abated, and ^contra: over this he 

 drew out a narrow glass cane, which he filled with a tinged spirit, and which 

 being about 1 5 times lighter than Mercury, would ascend about 15 times as 

 much as the Mercury in the barometer, fell. This, besides that tiie spirit 

 would dilate and contract itself with heat and cold, had the inconvenience of 



