4. What metals are used in the construction of Artillery? All heavy artillery, such as that for sea-coast, siege, and garrison equipment, is made of iron; and that for field service, of bronze. [Sidenote: 10]] 5. What is bronze for cannon? An ALLOY consisting of 90 parts of copper and 10 of tin, allowing a variation of one part of tin more or less. It is commonly called _brass_. 6. Why is bronze used in preference to iron, for field pieces? This metal, having greater tenacity and strength than iron, the pieces can be made lighter. 7. In what respect does iron merit a preference? Iron is less expensive than bronze, and is more capable of sustaining long-continued firing with larger charges; such pieces are, therefore, better calculated for the constant heavy firing of sieges. =NOTE.=—In the sieges in Spain, bronze guns could never support a heavier fire than 120 rounds in twenty-four hours, and were never used to batter at distances exceeding 300 yards; whereas, with iron guns, three times that number of rounds were fired with effect, from three times the distance, for several consecutive days, without any other injury than the enlargement of their vents. The comparative power of conducting heat in iron and copper being respectively as 3.743 to 8.932, taking gold at 10.000, it is evident that in practicing with iron and bronze pieces of the same calibre, it would soon become necessary to reduce the charges in the bronze pieces, and, also to increase the time between the discharges, to prevent their softening and drooping; while with iron, full charges and rapid firing may be kept up. 8. What additional objection has been urged to bronze for cannon? The difficulty of forming a perfect alloy, in consequence of the difference of fusibility of tin and copper. 9. What iron pieces are used in the land service? [Sidenote: 11]] 12, 13, and 24-pdr. siege and garrison guns, 32 and 42-pdr. sea-coast guns, 8-in. siege and 24-pdr. garrison howitzers, 8 and 10-in. sea-coast howitzers, 8 and 10-in. columbiads, 8 and 10-in. siege, and 10 and 13-in. sea-coast mortars. =NOTE.=—The 24-pdr. eprouvette is also of iron, and used for the proof of powder. 10. What are the kinds of bronze pieces in use at present? 6 and 12-pdr. field guns; 12-pdr. mountain howitzer; 12, 24 and 32-pdr. field howitzers; stone and 24-pdr. Coehorn mortars. 11. What is a battery? This term is applied to one or more pieces, or to the place where they are served. 12. What regulate the dimensions of cannon? The tenacity and elasticity of the metals employed in their fabrication. Their thickness must be proportioned to the effect developed by the powder; and the length is determined by experiment, and should not exceed 24 calibres. The exterior surface of a cannon is composed of several surfaces, more or less inclined to the axis of the bore, the forms of which have been determined by experiment.