Birch's law
Birch's law, discovered by the geophysicist Francis Birch, establishes a linear relation between compressional wave velocity vp and density ρ {\displaystyle \rho } of rocks and minerals: v p = a ( M ¯ ) + b ρ {\displaystyle v_{\mathrm {p} }=a({\bar {M}})+b\rho } where M ¯ {\displaystyle \,{\bar {M}}\,} is the mean atomic mass in formula units and a ( x ) {\displaystyle \,a(x)\,} is an empirical function determined by experiment. == Example == The mean atomic mass of forsterite (Mg2SiO4) is equal to the sum of the atomic masses divided by the number of atoms in the formula: M ¯ = 2 × ( 24.3 ) + 1 × ( 28.3 ) + 4 × ( 16.0 ) 2 + 1 + 4 = 20.13 .