information is conveyed from downhole location within a well , by varying the rate of flow of drilling fluid at the downhole location in accordance with changes in a predetermined condition at that location , and then sensing variations in the rate of flow of the drilling fluid at the surface of the earth as an indication of the downhole information being transmitted . a readout unit can be actuated in accordance with the information sensed at the surface of the earth to give a visual indication or other output representative of or dependent upon the downhole information .