a PHILOSOPHICAL TRANSACTIONS. [aNNO 1672. 



pressure, as they go and come through the whole extent of the mercury, and 

 that the glass does not hinder their communication with those that are without. 



To remove this difficulty, which in M. Huygens's own opinion is very great, 

 he answers, that though the parts of the matter, by him supposed, do find pas- 

 sage between those that compose the glass, quicksilver, &c; yet they there find 

 not sufficiently large ones for many to pass together, nor to move there with 

 that force which is requisite to separate the parts of the quicksilver, that have 

 some connexion together. And this very same connexion, he says, is the cause, ~ 

 that though on the side of the inner surface of the glass, which touches the sus- 

 pended mercury, many of its parts be pressed by the particles of this matter ; 

 yet there being also a great number of them that feel no pressure by reason of 

 the parts of the glass, behind which they are placed ; they retain one another, 

 and remain all suspended, because there is much less pressure on the surface of 

 the quicksilver that is contiguous to the glass, than upon that below, which is 

 all exposed to the action of that matter which makes this second pressure. 



The ingenious and candid author of this solution acknowledges himself, that it 

 does not so fully satisfy him as not to leave some scruple behind ; but then he 

 adds, that that keeps him not from being very well assured of that new pressure^ 

 which he has supposed besides that of the air, by reason as well of the experi- 

 ment already alleged, as of two others, which he subjoins, to this effect : — 



First, When two plates of metal or marble, whose surfaces are perfectly plain, 

 are put one upon another, they do so stick together, that the uppermost being 

 lifted up, the undermost follows without quitting it: and the cause thereof is 

 justly ascribed to the pressure of the air against their two external surfaces. He 

 taking then two plates, each of them but about an inch square, being of that 

 matter of which anciently they made looking glasses, and closing so exactly 

 together, that without putting any thing between, the uppermost keeps not 

 only up the other, but sometimes also with it three pounds of lead fastened to 

 the lowermost, and thus they remain together as long as you please. Having 

 thus joined them and charged them with three pounds weight, he suspended 

 them in the recipient of his engine, and exhausted it of air so far as that there 

 remained not enough to sustain by its pressure as much as an inch high of water; 

 and yet his plates disjoined not. He adds, that he made the same experiment 

 by putting spirit of wine between the two plates; and found, that in the reci- 

 pient evacuated of air they sustained, without being severed, the same weight 

 they did when it was full of air. This he thinks shows clearly enough, that 

 there remains yet in the recipient a pressure great enough after that of the air is 

 thence taken away ; and that there is no more reason to doubt of it, than of the 

 pressure of the air itself. 



