Ordinary unskilled laborers in the employ of the City shall be paid such rates of compensation, not exceeding the maximum rates prevailing in private employment in the City for like services, as may be established by rule of the Mayor. The rules of the Mayor establishing rates of compensation under this section may be revoked, altered, and amended at any time. Such rules may classify the unskilled labor positions in the City employment according to their character and may fix for each class or grade thereof an appropriate rate, by hour, day, week or month, as the nature of the case may require.