involved in tlie Construction of Artillery. 



177 



gun, creep up past both its flanks, traverse more or less over its upper surface, and 

 meeting, pass away. The diagram B illusti'ates this. 



81. Tlae conjoint action both of wind and of these 

 ascending currents will be to inflect the otherwise 

 vertical upward movement of the latter more or less 

 diagonally, but the conjoint action may produce more 

 rapid cooling than that of either separately. The 

 ascending currents of air, like those of the wind, evect 

 heat proportioned to the low temperature of the air and 

 their velocity of ascent. The latter is greatest at the 

 points of the gun that are hottest ; the velocity of the 

 wind being the same for every part of its length. 



82. The result as to cooling, therefore, is, that in moderately still air the 

 lower side of the gun, upon which the cold air first strikes, is cooled fastest, and 

 its top side slowest. The same is the case with a wind blowing in a line with the 

 axis of the gun ; but with a side wind the gun will be cooled fastest along a line 

 of its exterior, somewhere between its sections by a vertical and by a horizontal 

 plane, both passing through the axis, and will remain hottest along the side 

 diametrically opposite. 



83. The shot and gun being both iron, every degree of sensible heat, lost by 

 the former, will communicate a degree of sensible heat to the latter ; but the heat 

 lost is diffused through a larger mass, and hence conveys a diminished sense of 

 warmth. We have no means of determining, in the absence of experiments, 

 either the heat taken up by the gun, under given conditions in a given time, 

 or the actual velocity of its transmission through the metal forming the thick- 

 ness of the gun, from its interior surface. 



84. But we are enabled to show, that at whatever rate the interior surface of 

 the gun may be heated, the passage outwards of the heat through its mass will 

 be so slow and retarded that the interior must be always greatly hotter than 

 the exterior. The case is one of conduction, and may be viewed, without ma- 

 terial error, as analogous to that of a uniform metallic bar, heated at one end, 

 the bar being assumed as any elementary radial portion of the gun's thickness. 



8.5. Blot (Traite de Phys.) has shown that if the extremity of such a bar be 

 maintained at a temperature = (y) + F; y being that of the bar originally and 



