Spread of Liquids on Solid Bodies. 327 



horizontal glass plate under the siphon-mouth the drops may 

 be caused to fall upon different places of the plate, so that the 

 liquid forms shallow segments of spheres with sharp circular 

 edges. 



Fig. 1 shows the contrivance as it is used. Several liquids 

 may at the same time form drops near one another. The un- 

 used drops are caught by a square glass trough, upon the 

 upper cut edge of which a small strip of plate-glass G 2 is laid 

 to serve as a table. This table is set exactly horizontal with a 

 spirit-level and small wooden wedges. 



The glass plate with the drops is then placed upon a hori- 

 zontal rectangular plate of plate-glass, near which is set a ver- 

 tical divided circle, having a movable arm of light straw of 

 350 millims. length and carrying a sight of black paper. On 

 rotating the apparatus, the sight, which has an aperture of 2 

 millims. diameter, describes a vertical circle, in the centre of 

 which is the sharp edge of the drop. 



A luminous flame being placed at several metres distance, 

 two images are formed by reflexion, at the level surface of the 

 glass and at the curved surface of the drop, due to the reflect- 

 ing rays ARi and AR 2 (fig. 2). The first image is of 

 natural size, the second smaller in proportion as the fluid sur- 

 face is more curved. If the arm with the sight be turned 

 forward beyond the line A R 2 in which the last portion of the 

 surface of the drop reflects the light, the little image of the 

 flame suddenly disappears ; and this position is read off to the 

 exact minute of arc, with a vernier, upon the vertical divided 

 circle. The arm bearing the sight must be turned back through 

 an angle 26 in order to receive the image of the flame reflected 

 from the level surface of the plate. The last-named position is 

 determined once for all, and only verified as often as appears 

 necessary ; so that a single reading suffices to determine the 

 edge-angle 6. 



The exactness of this first method admits of being easily 

 further increased by the employment of a telescope ; yet I 

 have found the apparatus, in the simple form described, com- 

 pletely adequate for my rather long-sighted eye, so long as 

 values of 6 which do not exceed 40° are in question. 



For larger edge-angles, the method described has the disad- 

 vantage that the flame-images in the strongly- curved surface 

 of the drops are very small and, especially in daylight, difficult 

 to perceive. 



It is therefore more convenient to measure the edge-angle 

 by a second method, with a simple goniometer of the following 

 construction. 



Upon one side of a horizontal steel wire, A Aj (fig. 3) 



