562 PHILOSOPHICAL TRANSACTIONS. [ANNO 1700-1. 



the spirit-thermometer in all the degrees of heat and cold, noting at the same 

 time the precise height of the mercury in the common barometers, it will 

 readily be understood that whenever these two thermometers shall agree, the 

 pressure of the air is the same as it was when the air was included, and the 

 instrument graduated : that if in the air-thermometer the liquor stands higher 

 than the division marked on it, corresponding with that on the spirit-glass, it 

 is an indication that there is a greater pressure of the air at that time than 

 when the instrument was graduated ; and the contrary is to be concluded when 

 the air-glass stands lower than the spirit, viz. that then the air is so much 

 lighter,^ and the quicksilver in the ordinary barometers lower than at the said 

 time of graduation. And the spaces answering to an inch of mercury will be 

 more or less, according to the quantity of air so included, and the sinallness of 

 the glass tube, in which the liquor rises and falls, and maybe augmented almost 

 in any proportion, under that of the specific gravity of the liquor of the ther- 

 mometer, to that of mercury; so as to have a foot or more for an inch of 

 mercury, which is another greater conveniency. 



It has been observed by some, that in long keeping this instrument, the air 

 included either finds a means to escape, or deposits some vapours mixed with 

 it, or else by some other cause becomes less elastic, by which in process of 

 time it gives the height of the mercury somewhat greater than it ought ; but 

 this, if it should happen in some of them, hinders not its usefulness, for that 

 at any time may very easily be corrected by experiments ; and its rising and 

 falling are the things chiefly to be remarked in it, the just height being merely 

 a curiosity. 



In these parts of the world, long experience shows, that the rising of the 

 mercury forebodes fair weather after foul, and an easterly or northerly wind ; 

 and that its falling, on the contrary, signifies southerly or westerly winds, with 

 rain, or stormy winds, or both ; which latter it is of much more consequence 

 to provide against at sea than at land ; and in a storm the mercury beginning to 

 rise, is a sure sign that it begins to abate, as has been experienced in high lati- 

 tudes, both to the north and south of the equator. 



The form of this instrument is shown in fig. 13, pi. 13. Where ab repre- 

 sents the spirit-thermometer, graduated from O, or the freezing point, tlirough 

 all the possible degrees of the heat or cold of the air, at least in these climates. 

 CD is the air-thermometer, graduated after the same manner, with the like 

 degrees, ef is a plate applied to the side of the thermometer cd, graduated 

 into spaces, answering to inches, and parts of an inch, of mercury, in the 

 common barometers, g an irxlex standing on the plate at the height of the 

 mercury, as it was when the instrument was graduated, as suppose here at 



