VOL. XXX.] PHILOSOPHICAL TRANSACTIONS. 43* 



column of water below it, cb. From which it is plain, that no part of the 

 water contained in the tube can possibly descend, unless the upper part, assisted 

 by the weight of the water below it, be sufficient to overcome the attraction of 

 the annulus of glass at a. 



But in such a compound tube as that made use of in our experiment, fig. 4, 

 acb, the case is very different; and it does not easily appear, why in a vacuum 

 any part of the water in the wider part of the tube, as for example at c, should 

 not leave that which is above it, and descend ; since the annulus at c is by 

 much too wide to sustain a column of water of so great a length as cb. 



The best answer I can give to this difficulty is, that the cohesion between 

 the water contained in the capillary and that below it, is sufficient to balance 

 the weight of the column suspended. But how far this cohesion may depend 

 on the pressure of a medium subtle enough to penetrate the receiver, is worthy 

 of consideration. For though such a medium will pervade the pores of the 

 water, as well as those of the glass, yet it will act with its entire pressure on 

 all the solid particles, if I may so call them, of the surface of the water in 

 the cistern, whereas so many of the solid particles of the water in the 

 tube, as happen to lie directly under the solid particles of the water above 

 them, will thereby be secured from this pressure; and consequently there will 

 be a less pressure of this medium on any surface of the water in the tube 

 below the capillary, than on an equal surface of the water in the cistern. So 

 that the column of water suspended in the tube may be sustained by the differ- 

 ence between those two pressures. This explication seems to be favoured by 

 the following experiments, which may all be accounted for in the same manner, 

 though I shall soon mention another cause, which contributes to the success 

 of the first and second. 



The first is the famous experiment of the suspension of mercury, freed of 

 air, to the height of 70 or Jb inches, in the Torricellian tube, in the open 

 air: to which we may add the sustaining of mercury, likewise cleared of air, 

 within the exhausted receiver, as related by Mons. Papin, in his Continuation 

 du Digesteur. The next are the experiments made by M. Huygens, and de- 

 scribed in Phil. Trans. N'^ 86, on the cohering of polished plates with a con- 

 siderable force in the exhausted receiver; as also of the running of water and 

 mercury, when cleared of air, through a siphon of unequal legs in the 

 vacuum : all which he accounts for by the same principle, and much in the 

 same manner as we have used for explaining the experiment above. 



As to the existence of such a medium, I shall only refer to what has been 

 said by our illustrious president in the queries at the latter end of the last 

 edition of his Optics: and as I have lately exhibited before the society some 



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