1881.] 



On Pendent Drops. 



'611 



Thus if — 



r be the radius of the section, 



6 the inclination of the tangent to the axis of revolution, 

 H the head of liquid in centims., 



V the volume of the part below the section in cub. centims., 



T the surface tension in grams per linear centini., 



D the weight in grams of 1 cub. centini. of the liquid, 



then T cos 6 x 27rr=(Y+7rr 2 H)D, 



T _ (V + 7rr 2 H )D 

 2irr cos 



r and H were measured carefully with compasses and a millimetre 

 scale, which could be read by estimation to tenths of millimetres, and 

 cos 6 was determined directly by the same means after drawing the 

 tangent to the curve. V was obtained by dividing the area below the 

 section into narrow horizontal strips, whose dimensions were measured 

 with scale and compasses, and the volume of the solid generated by the 

 revolution of each, considered as the frustum of a cone, was calculated. 

 It was found in practice that the bottom part of the curve is generally 

 so nearly circular that the best way is to strike the circle of which it 

 is the arc, for other parts of the curve lines, ruled at a distance of 

 5 or 10 millims. apart (when the magnification was about fifteen times 

 linear), gave a sufficiently accurate approximation, it being impossible 

 to distinguish from straight lines the small portions of the curve inter- 

 cepted between each pair. 



It was easy, by marking the depth, to take two or more independent 

 tracings at exactly the same stage of extrusion, and superposition of 

 these showed very complete agreement, and gave confidence in the 

 result ; moreover, the symmetry of the tracing on the two sides of 

 the axis affords a check on the accuracy of the drawing. The tangent 

 is most easily drawn at a point of re-curvature, or where the curvature 

 in the plane of the paper is small ; on the other hand, an error in the 

 value of cos 6 increases in importance as increases. 



These considerations should influence the choice of sections. 



Treating in this way the curve of a drop of olive oil hanging in air, 

 the following values were obtained for the surface tension at the 

 different sections (1), (2), (3), and (4), which are arranged in order 

 of increasing area. 



T x = -049401, 

 T 2 =-051215, 

 T, = -0d7775, 

 T 4 = -065063. 



The discrepancy at once shows that the assumption of no pressure 



