"Of fundamental importance for the study of the physical properties of gases was the invention of the air-pump by , about the middle of the seventeenth century. ...The form of the instrument underwent a gradual evolution in his hands. The earlier types were of very simple design (see Illustr. [Fig. 1]). The first consisted of a cask well caulked with pitch and filled with water, which was evacuated by means of a brass pump having two valves. As the water was pumped out, however, the air was heard rushing through the pores of the wooden cask. A similar result was obtained when the cask was completely enclosed in a larger one, also containing water. Guericke accordingly gave up using wooden vessels, and attempted instead to evacuate a copper sphere from which he pumped out the air directly without previously filling it with water. ...[T]he sphere collapsed ...owing ...to the pressure of the external air, the vessel not having been made perfectly spherical. Guericke ...had another ...constructed ...and succeeded in obtaining a fairly high vacua. This must have been before 1654..."