50 PHILOSOPHICAL TRANSACTIONS. [aNNO 172^. 



gree of their respective scales ; the late Dr. Hook contrived an instrument, that 

 he called a marine barometer, made of a combination of the two above- 

 mentioned thermometers ; in such a manner, that a third scale being used to 

 observe the difference of the two thermometers, from which the change of the 

 air's gravity, and consequently storms, rains, and fair weather, might be fore- 

 told at sea, where the quicksilver barometer becomes useless by the motion o{ 

 the ship. 



Dr. Halley, some years since, published two tables, to show how much the 

 mercury in the barometer would subside, when the instrument is carried up to 

 determinate heights, above the level of the place where the first observation was 

 made ; but as he makes -j-l of an inch of fall of mercury to correspond with 

 only go feet in altitude, which is rather of the least, it is evident that only very 

 high hills and mountains can have their heights determined by this method. The 

 same learned professor has lately, in the Philos. Trans, proposed Mr. Patrick's 

 pendent barometer for taking the level of distant places, because the mercury 

 in that barometer will sometimes rise and fall a foot, or a foot and a half; if 

 therefore the motion of the mercury in this barometer, be 5 times more sensible 

 than in the common one, a 10th of an inch of fall of the mercury will answer 

 to a height of 18 feet; and therefore such an instrument might be of use in 

 taking the levels of distant places. But it is known by many experiments, that 

 this will not answer in practice ; because, as the tube of such a barometer is of 

 a very small bore, the attraction of cohesion, between the mercury and tube, 

 will disturb the motion of the mercury caused by the different pressure of the 

 atmosphere; so that setting up this barometer several times successively in the 

 same place, it will often differ a 10th of an inch, or more; and if it be shaken, 

 as is commonly done to set it right, the mercury will sometimes part, and a 

 drop of it fall from the rest : so that it is less to be depended on for this use 

 than the common barometer. 



Mr. Stephen Gray has often made a very sensible barometer in the following 

 manner. Into a bottle cb, fig. 7, pi- i, he fixes a tube ab, of a very small 

 bore, open at both ends, and cemented tight to the neck of the bottle at c ; 

 then having warmed the bottle with the hand to drive some of the air out of it, 

 he immerges the end a into water, tinged with cochineal ; so that as the air 

 cools in the bottle cb, some of the red water is forced into the bottle; then 

 setting the bottle upright again, as in the figure, the liquor in the bottle 

 will stand at b, above the end of the tube, and that in the tube at d ; but 

 if it should stand higher or lower than d, it may be brought to that place 

 by sucking or blowing at a. The instrument, thus prepared, being first set 

 on the ground, and a springing ring of fine wire slipped on the tube 



