438 PHILOSOPHICAL TRANSACTIONS. [aNNO I7I9. 



side of a wooden box. On the inside of this is another glass plane of the same 

 size, which at the end ac is pressed close to the former, and opens to a small 

 angle at the opposite end bd. When mercury is poured into this box to anv 

 height as ce, it insinuates itself between the two glass planes, and rising to 

 different heights between the glasses where tlie opening is greater or less, it 

 forms the common hy[>erbola cgf ; one of whose asymptotes ep is the line on 

 which the surface of the mercury in the box touches the inner glass ; the other 

 is the line ac, in which the planes are joined. This hyperbola being carefully 

 examined by Mr. Hauksbee and myself, the rectangle ehg, wherever taken, 

 proved always of an equal quantity, to as great an accuracy as could be expected, 

 when the planes were opened to any considerable angle : but when the opening 

 was very small, the inequalities of the planes, though the best 1 could procure, 

 bearing a greater proportion than before to the distance between them, oc- 

 casioned a sensible variation. Which I take to be the reason why the ordinates 

 found by the late Mr. Hauksbee, in examining the curve produced in a contrary 

 situation, on dipping two glass planes so joined into spirit of wine, do not 

 answer to those of the hyperbola. 



Exper. 4. — ab, fig. 8, is a perpendicular section through two glass planes 

 joined at a, and opened to a small angle at b. c represents a pretty large drop 

 of mercury, the larger the better, which, being made to descend as far as c, by 

 holding the planes in an erect position, with the end a downwards, retires from 

 the contact of the planes to d, on inclining the planes towards a horizontal 

 situation ; and the distance cd becomes greater or less, as the planes are more 

 or less inclined towards the horizon. 



A drop of any oily or watery liquor moves the contrary way ; as has been 

 shown by the late Mr. Hauksbee. 



Exper. 5. — ab, fig. Q, is a tube open at both ends, and a foot or two in 

 length, having its lower part drawn out into a fine capillary at b. This tube 

 being filled with mercury, the whole column of quicksilver will be sustained in 

 it, provided the capillary tube at b be sufficiently small. But if the mercury in 

 the end b be sufi'ered to touch any other mercury, it runs all out of the tube. 

 If, without letting it touch any other mercury, a small part of the end b be 

 broken oflT, the mercury will run out, till it comes to some lesser height as bc, 

 at which it will again stop, the height bc being nearly in a reciprocal propor- 

 tion to the diameter of the small end of the tube. The 7th experiment in 

 N° 355 is the reverse of this. 



Exper. 6. — Fig. 10 is the same in substance with the former, but made with 

 a large glass funnel ab, instead of a tube. The reverse of this in water is the 

 13th experiment in the same N*^. 



