Smeaton, invented in 1756, studied the effects of different limestones in the three-component mortar of "lime. volcanic. gray sand" on the properties of the mortar. He found that limestone containing clay was calcined and finely ground. Finally, the mortar made by adding water can slowly harden. Its strength in seawater is much higher than that of "Roman Mortar" and it can withstand the erosion of seawater. Smeaton used the newly discovered mortar to build the world-famous Eddystone Lighthouse in Plymouth Harbour.