The bright spots had a velocity profile shown in Fig. 6(b) which gives them an average velocity of approximately 0.4 m/s. This is in agreement with the findings of Hirano and Fabbro [17] who found a hump velocity of 0.2 m/s. If the bright spots were correlated with the surface velocity of the melt, values between 1.5 and 2.0 m/s should be expected (i.e.assuming the velocity of the melt surface would equal the highest values noted in Fig. 5).