The half-cone of population yielded by model (6.4) is illustrated in Figure 6.24. Mills (1969) has shown that the exponential decay in residential density is a function of increase in total transport costs with distance from the city centre. Direct studies of the term, P0, in equation (6.5) have been few, largely because central density has proved so difficult to define. Winsborough (1961) overcame this problem obliquely by demonstrating that central density is a function of the overall population density of a city, regardless of its density gradient. Since the overall population density of a city is significantly associated with the age of dwellings, city size, and the extent of manufacturing employment, so central density will also vary with these attributes