By the late 1940s it was known that the properties of atomic nuclei could not be explained by the then-current models such as the liquid drop model developed by Niels Bohr amongst others. The shell model, developed in 1949 by Maria Goeppert-Mayer and others, allowed some additional features to be explained, in particular the so-called magic numbers. However, there were also properties that could not be explained, including the non-spherical distribution of charge in certain nuclei.[17] In a 1950 paper, James Rainwater of Columbia University suggested a variant of the drop model of the nucleus that could explain a non-spherical charge distribution.[18] Rainwater's model postulated a nucleus like a balloon with balls inside that distort the surface as they move about.