502,127. Air-filters. GRESHAM & CRAVEN, Ltd., and GRESHAM, J. N. Dec. 8, 1937, No. 33950. [Classes 8 (i) and 8 (ii)] An air-filter a used in conjunction with air compressing or exhausting apparatus is combined with a valve k controlling an apertured cover plate n. The filter when in position holds this valve closed ; if the filter should be absent, the valve opens allowing inlet of air which interferes with the operation of the apparatus and thus notifies the attendant of its absence.