A tidal model has been introduced to the triple system of the Galaxy, Large and Small Magellanic Clouds (the LMC and SMC hereafter) and successfully reproduced the Magellanic Stream (Murai and Fujimoto 1980; Lin and Lynden-Bell 1982; Gardiner et al. 1994; Gardiner and Noguchi 1995; Lin et al. 1995), a narrow band of diffuse atomic hydrogen gas emerging from the SMC region, passing by the South Galactic Pole along an overhead great circle spanning over 100° (Wannier and Wrixon 1972; Mathewson et al. 1974). The LMC and SMC have a hydrogen bridge and common envelope (Hindman 1964; McGee and Milton 1966) and, therefore, we can consider that they have been in a binary state for the Hubble time, revolving together around the Galaxy with a halo whose mass is larger than 1012
⊙ if the flat rotation curve extends up to more than 100 kpc. The strong gravitational force due to this heavy halo attracts the Magellanic Stream and produces the high negative radial velocities (Murai and Fujimoto 1980).