4l6 PHILOSOPHICAL TRANSACTIONS. [aNNO' 1 67O. 



Observations on the Baroscope and Tliermoscope. By Dr. JVallis, in 

 a Letter of January 7, 166*9-70. N'' 55, p. 11 16. 



Whereas I formerly observed, that in hot weather the quicksilver in the 

 baroscope used to rise observably, especially in sun-shine and the heat of the 

 day; which might seem to argue the air to be thereby made heavier; I now 

 find, having kept the same barometer for the space of five years unaltered, the 

 case for these two years last past, to be somewhat otherwise : and that in hot 

 sun-shiny weather the quicksilver rather subsides a little ; and in extreme 

 cold and frosty weather it rises. Which makes me judge the cause of these 

 contrary observations to be this, viz. That the quicksilver, at its first putting 

 into the tube or baroscope, was not perfectly cleansed from the air : which 

 latent particles of air, by the external heat, were so much expanded as to give 

 a greater bulk to the same quantity of quicksilver, with which it was mingled, 

 and consequently to make it rise somewhat higher, and on the recess of the 

 external heat, the spring again slackening, the air being more compressed, 

 suffered the quicksilver to be again contracted into its former lesser dimensions, 

 and so to become heavier, and not to rise so high as before, when it was hot- 

 ter. But now, the air freed from its entanglement with the quicksilver, being 

 got up into the void part of the tube above the quicksilver, acts contrarywise, 

 that is, when it is by heat expanded, it presses downward on the quicksilver, 

 and a little depresses it ; and on the contrary, when by frost or very cold wea- 

 ther this air is contracted, the quicksilver, freed from that pressure, rises a 

 little. 



My thermoscope, or sealed weather-glass, has this last frost been much lower 

 than I ever have known it on five years constant observation ; which pro- 

 ceeds partly from the extremity of the cold more than ordinary, and partly 

 from the inclosed liquor (being spirit of wine tinged with cochineal,) growing 

 less spirituous. It was first made in December [664. In the months of 

 January and of February following, we had very smart frosts, more cold than 

 ordinary ; when yet the lowest mark to which the liquor did subside in very 

 hard frosts and very cold wind, was at 12^ inches : the height in summer fol- 

 lowing, 1665, was usually at 20, 21, 22, or thereabout; but in some few 

 very hot days, at 25, 26, 264^ ; the whole height of the small cylindric glass, 

 whose cavity was about -f of an inch diameter, being about 20 inches ; besides 

 a small spherical bowl at the top, of about -f- of an inch diameter ; and a bowl 

 at the bottom, which contained the liquor of about two inches diameter : the 

 space above the liquor being at the first composure of it, void of air, save 



