Black Tinder.—­Tinder that is black by previous charring, or from any other cause, ignites in the sun far sooner than light-coloured tinder.

Fire by conversion of motion into heat.—­General Remarks.—­When a moving body is arrested, heat is given out; the quantity of heat being in exact proportion to the mass, multiplied into the square of its velocity.  Thus if a cannon ball be fired at an iron target, both it and the ball become exceedingly hot.  There is even a flash of light when the velocity of the ball is very high.  When bullets are fired with heavy charges at a target, the lead is just melted by the heat of impact, and it “splashes,” to use a common phrase.  It is obvious from these two examples, that no velocity which the hand of man is able to give to a steel, when striking a flint, or to one stick rubbing against another stick, will be competent to afford a red-hot temperature unless the surface against which impact or friction is made be very small, or unless great care be taken to avoid the wasteful dissipation of heat.  The spark made by a flint and steel, consists of a thin shaving of steel, scraped off by the flint and heated by the arrested motion.  When well struck, the spark is white-hot and at that temperature it burns with bright scintillations in the air, just as iron that is merely red-hot burns in pure oxygen.  This is the theory:  now for the practice.

Flints.—­If we may rely on a well-known passage in Virgil, concerning AEneas and his comrades, fire was sometimes made in ancient days by striking together two flints, but I confess myself wholly unable to light tinder with flints alone, and I am equally at a loss to understand what were the “dry leaves” that they are said in the same passage to have used for tinder.  Neither can I obtain fire except with a flint and steel, or, at least, hardened iron; a flint and ordinary iron will not give an available spark.  Flints may be replaced by any siliceous stone, as agate, rock-crystal, or quartz.  Agate is preferred to flint, for it gives a hotter spark:  it is sold by tobacconists.  A partly siliceous stone, such as granite, will answer in default of one that is wholly siliceous.  I have been surprised at finding that crockery and porcelain of all kinds will make a spark, and sometimes a very good one.  There are cases where a broken teacup might be the salvation of many lives in a shipwrecked party.  On coral-reefs, and other coasts destitute of flinty stones, search should be made for drift-wood and drifted sea-weed.  In the roots of these, the pebbles of other shores are not unfrequently entangled, and flint may be found among them.  The joints of bamboos occasionally contain enough silex to give a spark.

Steels.—­The possession of a really good steel is a matter of great comfort in rough travel, for, as I have just said, common iron is incompetent to afford a useful spark, and hardened iron or soft steel is barely sufficient to do so.  Any blacksmith will make a good steel out of an old file, if he has nothing more appropriate at hand.  A substitute for a steel can be made, even by an ordinary traveller, out of common iron, by means of “casehardening” (which see).  The link of a chain, or the heel of a boot, or a broken horse-shoe, is of a convenient shape for the purpose.