The physical characteristics of blue horizontal-branch stars in the northern globular clusters MIS, M92, Ni 113, and M3 are investigated on the basis of Sandage's (1969) UBV observations. These data are interpreted by means of the empideal correlations between broad-band colors and atmospheuc parameters found for the blue horizontal-branch stars of NGC 6397 and w Cen by Newell, Rodgers, and Searle. Use is made also of Sargent's low-dispersion spectra of several stars in NIlS and M92. A discussion of the results for the four northern clusters, together with the results pi eviously obtained for NGC 6397 and w Cen, leads to four principal conclusions: i) The temperature-color relations defined by blue horizontal-branch stars with T < 10000 K differ from those of the corresponding Population I stars. This effect is caused by differences in metal abundance and surface gravity. ii) Blue horizontal-branch stars have mass-luminosity ra tios of log = -2.0 t 0.1. iii) The evolution of stars described by Iben and Faulkner's models of high helium abundance (F 0.35) and a double energy source cannot satisfactorily ezplain the blue horizontal branch in all six clusters. iv) Helium is underabundant by a factor of at least 10 on the surface of star S-iS in MiS.