The mortar shells of that era were basically fragmentation bombs shaped like flying saucers. The fuses were hollow quills filled with powder and calibrated to burn at a specific rate. If a bombshell arrived too soon either due to too long a fuse or defective powder, there might have been a significant length sticking out of it. The fuse had to have been burning actively or Hornblower would not have ruined his knit gloves when he snuffed it.