Principally to light artillery, in firing shells and particularly spherical case, where regularity and certainty are essential requisites. [Sidenote: 114]] 14. Mention one important advantage of this fuze. The shells can be loaded, all ready for use, and remain so any length of time, perfectly safe from explosion, as the fuze can be screwed into its place, and the composition never exposed to external fire until the metal is cut through. 15. What is the only operation under fire required? To gouge through the metal at the proper point, with any kind of chisel, knife, or other instrument. 16. Describe the United States sea-coast fuze. In the United States, a bronze fuze-plug has been adopted for heavy shells instead of the wooden one. It fits the eye in the same way, and is retained by friction. It having been found that ricochets, especially over water, were apt to extinguish these fuzes, a safety cap and primer combined have been adopted in the navy. A recess in the top is filled with priming composition and covered, until the fuze is required for use, with a leaden disk which fits accurately the opening. A crooked passage filled with priming, conveys the fire to the fuze composition beneath, and prevents water from being forced in in sufficient quantity to extinguish the fuze. 17. When are paper fuzes for field shells and spherical case inserted? At the moment of loading the gun, and into wooden _fuze-plugs_ previously driven into the shell. 18. What is a _port-fire_? It consists of a small paper-case filled with a highly inflammable but slow burning composition, the flame of which is very intense and penetrating, and cannot be extinguished by water. [Sidenote: 115]] 19. What is it used for? Principally as an incendiary material in loading shells, and for communicating fire to the priming of guns when proving them. 20. What does port-fire composition consist of? Of nitre, sulphur, and mealed powder, in different proportions. One kind is composed of Nitre, 65 parts. Sulphur, 22.5 “ Mealed powder, 12.5 “ A port-fire case, eighteen inches in length filled with this composition, burns ten minutes. 21. What are _priming-tubes_, and their use? Small pipes having a cap at one end, and filled with a composition for firing cannon. 22. What tube is in general use in our service? The friction primer. 23. Describe it.