Contact angle of liquid drop on solid surface.

[amk_dbz]

Can anyone explain point 44 of the attached pdf document on surface tension.

(Here's the link in case attachment doesn't work:
http://www.sakshieducation.com/EAMCET/QR/Physics/Jr Phy/12Surface tension _198-208_.pdf)

How is the surface tension direction found out?
Also why is surface tension for Solid-Air Surface considered when balancing forces on liquid?
Any help will be appreciated
Thank you.

 

[andrien]

direction will be tangential to both interfaces.