| 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.
It consists of a short tube of metal inserted into a hole near the top of a longer tube, and soldered in that position. The short tube, is lined with a composition made by mixing together two parts of chlorate of potassa and one of sulphurate of antimony, moistened with gum water. A serrated wire passes through the short tube and a hole opposite to it in the side of the long one, the open end of the short tube being compressed with nippers, and the wire at the end of the serrated part doubled under to prevent any displacement. The other end of the wire is doubled and twisted by machinery. The long tube is filled with musket powder, its upper end being closed with shellac-varnish, and its lower with shoemakers-wax.
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24. What advantage does the friction tube possess?