1) Rayleigh's Method:- When expression for a variable which depends upon maximum three or four variables only has to be determined this method is used. In this, if the no. of independent variable exceeds more than 4, then it is very difficult to find the expression for the dependent variable.

Let 'X' be a variable which depends on $x_{1},x_{2} \ and \ x_{3}$ variables, then according to Rayleigh's method 'X' is a function of $x_{1},x_{2} \ and \ x_{3}$

$x=f[x_{1}, x_{2}, x_{3}]$