The Sward Hardness Rocker has been used for many years to measure a property of thin paint films called the Sward hardness. This paper shows that Sward Hardness Rocker measures the modulus of elasticity of those films. It shows how the glass upon which the Sward Rocker is standardized compares with other materials of the same dimensions. This paper also shows the serious effect a variation in temperature will make on the Sward hardness and the subsequent modulus.