From the PREFACE. The experiments of the present volume are either direct applications of displacement interferometry or they embody the correlative work which has grown out of such applications, often at widely different times. In the arrangement of the chapters it has therefore been expedient to depart from chronological order in favor of an arrangement of subjects which belong together. In a former report I had already used a U-tube in connection with the interferometer, but the design of the apparatus was limited in scope. In the present paper (Chapter I) the open mercury manometer is made directly available for pressure measurement, and as the attainable sensitiveness is easily a few hundred thousandths of a centimeter of mercury per fringe displacement, it is well worth while to see what can be gained by using it. The applications to air thermometry on a micrometric scale and an attempt to revive the old absolute electrometer in Chapter II are merely incidental....