Edit 1: Kudos to “gjm” (see above) for pointing to Spivak’s page on Amazon!
Edit 2: Spivak’s Hogwarts proof implicitly uses a fundamental theorem in differential geometry that is called Cartan’s Magic Formula … this oblique magical reference is Spivak’s joke … as with many magical formulas, the origins of Cartan’s formula are obscure.
Regrettably, tgb, even the redoubtable Google Books does not provide page-images for Spivak’s Physics for Mathematicians: Mechanics I. The best advice I can give is to seek this book within a university library system.
Edit 1: Kudos to “gjm” (see above) for pointing to Spivak’s page on Amazon!
Edit 2: Spivak’s Hogwarts proof implicitly uses a fundamental theorem in differential geometry that is called Cartan’s Magic Formula … this oblique magical reference is Spivak’s joke … as with many magical formulas, the origins of Cartan’s formula are obscure.