Little models of houses, masts of ships, trees, and towers are sold by the instrument makers, and by placing a long balanced wire on the top of the pointed wire of a large Leyden jar, having one end furnished with wool to represent a cloud, a most excellent imitation of the effects of a charged thunder-cloud is produced. The mechanical effect of a flash of lightning has been analysed, and it has been stated, in one instance, that the power developed through fifty feet was equal to a 12,220 horse-power engine, or about the power of the engines of the _Great Eastern_, and that the explosive power was equal to a pressure of three hundred millions of tons. (Fig. 177.) [Page 192] [Illustration: Fig. 177. A. Charged Leyden jar with balanced wire and wool at B, representing a thunder-cloud. C. The obelisk overturned with the discharge. D. Another model of the gable end of a house; the square pieces of wood fly out when the continuity of the conductor is broken.] It was the learned but humble minded Dr. Franklin who established the identity between the mimic effects of the electrical machines (such as have been described), and the awe-inspiring thunder and lightning of nature. A copper rod, half an inch thick, pointed and gilt at the extremity, and carried to the highest point of a building, will protect a circle with a radius of twice its length. The bottom of the rod must be passed into the earth till it touches a damp stratum. [Illustration: Fig. 178. A storm.] [Page 193] CHAPTER XIV. VOLTAIC ELECTRICITY. In describing the various means by which electricity may be obtained, it was stated that "Chemical Action" was a most important source of this remarkable agent; at the same time it must be understood that it is not every kind of chemical action which is adapted for the purpose; there are certain principles to be rigidly adhered to--first, in the generation of the force; and secondly, in carrying it by wires so as to be applicable either for telegraphic purposes, or for the highly valuable processes of electrotyping and electro-silvering, plating, and gilding.