Steam digester Torricelli and further developed by German scientist Otto von Guericke with his Magdeburg hemispheres. Guerike's demonstration was documented by Gaspar Schott, in a book that was read by Robert Boyle. Boyle and his assistant Robert Hooke improved Guericke's air pump design and built their own. From this, through various experiments, they formulated what is called Boyle's law, which states that the volume of a body of an ideal gas is inversely proportional to its pressure. Soon the ideal gas law was formulated. Based on these concepts in 1679 a Boyle's associate, named Denis Papin, built a "bone digester", which is
Theory of intervening opportunities Theory of intervening opportunities Theory of intervening opportunities attempts to describe the likelihood of migration. Its hypothesis is that this likelihood is influenced most by the opportunities to settle at the destination, less by distance or population pressure at the starting point. Stouffer's law of intervening opportunities states, ""The number of persons going a given distance is directly proportional to the number of opportunities at that distance and inversely proportional to the number of intervening opportunities."" Stouffer theorises that the amount of migration over a given distance is directly proportional to the number of opportunities at the place of destination,