OBSERVED BY DR. YOUNG. 583 
of the latter must be the same as well in the parts parallel to the 
sides as in the upper part, in contact with the superimposed 
fluid, which part will consequently join with the former, bend- 
ing downwards in a direction tangent to the sides, and we shall 
have »=0. If on the contrary it is the lower fluid that wets 
the sides, the tension in the surface of contact of the two liquids 
will also be constant, but this surface will bend itself in a direc- 
tion tangent to the sides, turning its concavity upwards, and 
consequently we shall have #,=7. These results agree with 
those that Poisson (Art. 73) has deduced from other principles. 
5. Having established the rule to be followed in assigning the 
value of w,, we must, in order to possess all the numerical data 
to be introduced into equations (a.), know the value of T,, which 
represents the tension of the water in contact with the oil. To 
obtain this I shall avail myself of an experiment which our 
illustrious member Cavalier Avogadro has published in vol. xl. 
of the Memorie dell’ Accademia di Torino, and in the Annales de 
Chimie et Physique, Avril 1837. 
After coating with oil the inside of a small glass tube of rad. 
= 1 millimetre, he introduced it vertically into a trough of 
water. The water rose in the tube to the height of 5:34 milli- 
metres, raising on its upper surface a slender stratum of oil 
which it detached from the sides as it rose. The specific gravity 
of the oil employed was A = 908, that of water being taken as 
unity. The tension of the free surface in contact with the air 
of this oil, calculated according to the theory by an experiment 
of the same author, would be T = 3°81gA. 
Now if we observe that in this experiment the upper surface 
of the oil was tangent to the sides with its concavity turned 
upwards, and that of the lower fluid or water must according to 
the exposed principles be at its circumference also tangent to 
the direction of the sides, but with its concavity turned down- 
wards, we shall have w=, w,=0, and therefore from equa- 
tions (2.) we get 
T= —T, lr, = T,. 
Substituting these values in the second equation (a.), we obtain 
