Hi, for the proof you can refer to Fluid mechanics by R.K Bansal or see it from the link i am attaching here (see page 314 & 315)...if you dont get to understand after seeing this from the book, then do comment here.. i will get back to you : http://books.google.co.in/books?id=nCnifcUdNp4C&printsec=frontcover&source=gbs_ge_summary_r&hl=en#v=onepage&q&f=false