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Summary Analysis of the observations of the variations in the Earth's speed of rotation reveals the usual semi-annual, annual and long-period terms. In addition these observations indicate the existence of a biennial term whose behaviour is intermittent. The geophysical and meteorological excitation functions have been evaluated and they are in very good agreement with the observed astronomical variations. The principal phenomena contributing to the total excitation function is the global zonal wind circulation up to altitudes of about 30 km. The irregular behaviour of the biennial term in the astronomical data enables some conclusions to be drawn about the biennial zonal wind characteristics. It appears that in 1960 and 1961 the downward propagation of the biennial winds was less pronounced than normal and that in the interval 1964 to 1968 the biennial wind component had either a period of about three years or its behaviour was confused at lower altitudes. From 1969 onwards the astronomical data indicates the existence of a strong biennial wind component propagating significantly down to altitudes of perhaps as low as 12 km.