THE Research Institute of National Defence in Sweden operates a microbarograph at 59° 20′ N. and 18° 6′ E. in Stockholm. The instrument is a modification of the Shida type1,2 with four pressure intakes within a radius of 25 m. The main characteristics include: a minimum discernible pressure variation of 0.5 µbar, a sensitivity relative middle frequencies of 3 db. at 0.01 and 0.5 c./sec., and a stability of better than 5 µbar/24 hr.