The City Point explosion was caused by a "horological torpedo" as it was called at the time, but are there any details about the mechanism? Some sources have credited a clockwork device, but a former Confederate in 1875 described a "volcanic fuse" in which the delay was caused by acid eating through a copper cap covering the priming compound (similar to the thermite pencils used by the OSS is WW II). Are there any primary sources that are reasonably definitive?