The first simple classification of earthquake intensity was devised by Domenico Pignataro in the 1780s. The first recognisable intensity scale in the modern sense of the word was drawn up by P.N.G. Egen in 1828. However, the first modern mapping of earthquake intensity was made by Robert Mallet, an Irish engineer who was sent by Imperial College, London, to research the December 1857 Basilicata earthquake, also known as The Great Neapolitan Earthquake of 1857. The first widely adopted intensity scale, the "Rossi–Forel scale", was introduced in the late 19th century as a 10 grade scale. In 1902, Italian seismologist Giuseppe Mercalli, created the Mercalli Scale, a new 12-grade scale. A very significant improvement was achieved, mainly by Charles Francis Richter during the 1950s, when (1) a correlation was found between seismic intensity and the Peak ground acceleration - PGA (see the equation that Richter found for California). (2) a definition of the strength of the buildings, and a subdivision into groups (called type of buildings) was made. Then, the evaluation of the seismic intensity was based upon the damage grade to a given type of structure. That gave the Mercalli Scale, as well as the followed European MSK-64 scale, the quantitative element, which represents the vulnerability of the building's type. Since then, that scale was called the Modified Mercalli intensity scale - MMS and the evaluations of the Seismic Intensities became more reliable.