The wakes of circular cylinders were explored by means of hot wire anemometers in the range of Reynolds numbers based on diameter of from 10$^3$ to 4 x 10$^4$. The vortex strength was determined and its variation with distance downstream and with Reynolds number was investigated. The vortex strengths agree with those obtained by application of Roshko's (1954) method to our mean flow measurements. The vortex velocity distribution assumed was that of Hoffman & Joubert (1963) which was found to agree with our measurements when slight adjustments were made in the inner core region. Vortex paths, the formation region and three dimensional effects are also discussed.