640. JOHN A. DAHLGREN (1809-1870) Union rear admiral and inventor of the cannon which bore his name. He also commanded the South Atlantic Blockading Squadron. Fine content war-date A.L.S., 1p. 8vo., aboard the Philadelphia, Jan. 17, 1864 to an unnamed admiral. In part: "...I have your note in regard to Carter, and ordered the examination he asks for. A great desideratum is the sonic compass for the Monitors - that can be used in battle as well as on other occasions - the want of it limits the actions of these vessels sadly..." Very good. We can find no reference for a "sonic compass", but must assume that it was a compass with a dampening device to counter the effect of the tremendous shock caused by cannon recoil on solid steel vessels. $300-400