ADHESION OF BODIES TO FLUIDS. ] "7 J 



disk. The weight of this column, in a state of equili- 

 brium, is equal to that of a fluid cylinder, the base of which 

 is the surface of the disk, and the altitude the product of 

 one millimetre, multiplied by the number of millimetres the 

 fluid would sink in a tube of the same material as the disk, 

 and of one millimetre in diameter, this product being mul- 

 tiplied by the line of half the acute angle that the surface 

 of the fluid forms with the disk, and divided by the square 

 root of the cosine of the whole angle. 



All these results require a slight correction, relative to the 

 supposition of a great diameter of the disk. I shall give 

 this correction, which may be neglected without any sensi- 

 ble error for disks the diameter of which is thirty millime- 

 tres [11*74 lines] and upward.. 



To compare the preceding results with experiment, let us The theory 

 consider a disk of glass 100 millimetres [3 in. 9 1.1 in diame- compared with 



i» T tt i i -i i • t experiment. 



ter. Mr. Hatiy has observed, that m a tube of glass one 



millimetre in diameter, water would rise 13.569 millimetres 

 above the level : whence it is easy to conclude, by means of 

 the theorem above given, that the force necessary to sepa- 

 rate the disk from the surface of water would be equal to a 

 weight of 28*931 grammes [446*819 grs.]. Now according 

 to Mr. Achard this force is 29*319 grammes [452*832 grs.], 

 which differs very little from the preceding result. I made 

 some experiments on the resistance opposed by a disk of 

 glass applied to the surface of mercury. But to compare 

 them with the theory, it is necessary to know the angle 

 formed by the surface of this fluid in contact with the glass. 

 An experiment of this kind, made with precision, is well 

 adapted to determine this angle, which.appears to be of 30 g 

 or 40°. 



If we place two disks of glass horizontally on each other, Two disks of 

 leaving between them a very thin stratum of water, these f^aTum^of * 

 two disks will adhere with considerable force. To determine water between 

 this force, it must be observed, that the interposed fluid them >' 

 then takes the form of a pulley ; and that the smallest ra- 

 dius of curvature of its surface is very nearly equal to half 

 the thickness of the stratum. Neglecting here then, as may 

 be done when the disks are very large, the greatest radius of 

 curvature, we find the resistance, that the two cylinders op- 

 N 2 pose 



