Page:Encyclopædia Britannica, Ninth Edition, v. 14.djvu/656

 634 L I G L I G and whose semivertical angle is about 45. This is pro bably not true if the point be very high, on the top of a tall chimney or tower, for instance. Objects not far from the base of such a protected tower, and within the cone just described, have occasionally been damaged by lightning. The second condition is easily fulfilled in towns by connecting the lower end of the rod with the iron gas and water-mains, which form an excellent &quot; earth,&quot; as it is technically called. Water-pipes, being usually jointed with metallic-lead-washers, are preferable to gas-pipes, which are usually put together with white lead. This condition is also easy to secure in ships and in lighthouses, where large metal plates (in the case of a ship, the copper sheathing is precisely what is required) can easily be permanently immersed in sea-water. In country houses it is usually more difficult to obtain a proper earth. Plates and tubes of metal, of large surface, buried in ground which is permanently damp, form usually the best arrange ment. A well makes a good earth; a carefully constructed water tank (of stone or cement) is not an earth at all. The third condition, so far as experience can guide us, seems to be effectually realized by making the conductor throughout of iron rod of an inch in diameter, or of copper rod not less in diameter than fths of an inch. Such rods of equal length have nearly the same conducting power, and therefore would have equal amounts of heat developed in them by a given discharge. But if such a discharge took place, the copper would be heated much more than the iron, in consequence of its smaller mass per foot (the specific heat being approximately the same in the two materials). Hence iron is, in this respect, preferable to copper, if the conducting powers of the rods are equal. Another advantage possessed by the iron rod is that it is much less likely to be wilfully damaged or stolen. Against this may be set the objections that it is easily injured by rust, and is not nearly so flexible as the equivalent copper rod. Conductors are now usually made of wire-rope, so that the question of flexibility is no longer of serious importance ; but when iron is used it should always be protected by zinc, i.e., be what is absurdly called &quot;galvanized.&quot; Many fantastic forms of lightning rods were devised in consequence of the old erroneous notion that their efficiency depended on their surface and not on their cross section. In reality all conductors of equal length, and of the same material, are equally efficient if their cross sections be equal. 1 Thus, instead of stating the diameter of a rod, we may speak of its weight per foot, and say that a copper conductor should weigh at least about half a pound, and an iron one at least two pounds and a half per foot, provided the materials be of good conducting quality. The points need not be very sharp, but they ought to be protected by a coating of platinum or other non-oxidizable metal. And they should be in a group of two or three at the end of each branch of the rod, lest one of them should be fused and impaired in efficiency by an accumulation of electricity so rapid as to make the silent continuous dis charge impossible. Joints should be avoided as far as possible ; where they are unavoidable they should be made, not by screws or brazing, but by means of a large mass of solder completely enveloping the ends to be connected. Another point to be carefully attended to is that all large metallic bodies, such as lead or zinc roofing, metal tanks, &c., should be in good conducting connexion with the rod, so as to prevent discharges of electricity inside the ship or building. In many buildings we see the lightning rods 1 Some curious modifications of this statement are introduced when we deal with magnetizable metals, but they are unimportant in practice. attached by means of glass or porcelain insulators, such as are employed for telegraph wires. This is a perfectly needless, expensive, and possibly dangerous practice. The literature of this subject is very extensive, as may be seen from Ronalds s Catalogue of Works on Electricity. The reader may also consult Anderson on Lightning Conductors (1880), and the Report of the Lightning-Mod Conference (1882). In the latter work will be found abstracts of many valuable papers, especially the reports on lightning-rods made to the French Academy by some of its most distinguished members, including Coulomb, Laplace, Poisson, Gay Lussac, Fresnel, Pouillet, Cagniard de la Tour, Regnatilt, &c. There will also be found hints about some of the most ludicrous devices employed by men ignorant of the laws of electrical pheno mena. One of themost singular of thesewas theso-called &quot;llcpeller.&quot; A lightning-rod, in all respects sufficient, was wont to be capped by a piece of glass like a thick soda-water bottle, inverted upon its point. The effect of this could only have been to prevent the possibility of the silent discharge, to produce which is the proper function of the rod, and to make probable a lightning flash, just as if the rod had been- terminated by a ball instead of a point. One of these dangerous monuments of ignorance was removed from a British lighthouse within the last ten years. In an Irish light house, which was recently examined after suffering serious damage, it was found that the lower end of the lightning-rod was jumped into the solid rock a truly original form of earth ! In 1876 Clerk Maxwell suggested to the British Association the idea (based on Faraday s experiments) of protecting a building from the effects of lightning by surrounding it with a sort of cage of rods or stout wire. Here an &quot;earth &quot; would not be absolutely required. The present writer had some months previously suggested the same idea in a Eeport to the Board of Northern Lights. It is possible, though not certain, that this form of defence might be useful against globe-lightning, which xindoubtedly occurs, and against which ordinary lightning-rods would probably be of little use. These brief remarks contain all that is yet known to be necessary to the complete solution of an important practical problem about which many treatises have been written. (P. G. T.) LIGNITE. See COAL. LIGNUM VITM See GITAIACUM. LIGUORI, LIGUORI AN ISM. The name Liguorian- ism has been popularly given in the present century to a particular school of moral and devotional theology in the Roman Catholic Church by the controversial oppo nents of that school, whether themselves Roman Catholics or not. It is derived from the name of one of its principal and most influential exponents, Alfonso Maria de Liguori, a theologian, saint, and doctor of the Roman Church. In strictness, the term is not accurate, for Liguori was in no sense the founder of the school, nor did he innovate upon, develop, or exaggerate its principles and maxims. He was simply a fair representative of the national type of piety of Italian devotees in his day ; and, as a casuist, he was a diligent compiler, whose avowed design was to take a middle course between the two principal varieties of teaching in moral theology current in his own time, avoid ing their extremes of severity or laxity. His own words, in the preface to his Homo Apostolicus, a work intended for the guidance of priests in hearing confession, explain clearly the intention of his bulkier treatise, the Theologia Moralis. He says : &quot; When compiling that work, I spent the labour of fifteen years in perusing and weighing the teaching of very many writers whom I examined, some of whom I found more lenient than is just ;. . . while I found others who, strongly disliking such indulgence, fell into the other extreme of excessive rigour. And this was my principal task, to select from such a mass of opinions those decisions which, on the one hand, should uphold the obedience due to the precepts of God and of the church, and on the other should not add burdens which God has not imposed, by binding every one to that perfection which, through human weakness, is morally impossible to the general body of believers.&quot; A brief glance at the names of those casuists whom he cites most frequently, as Covarruvias, Soto, Lessius, Vasquez, Bonacina, the doctors of Salamanca, Sanchez, Diana, &c., shows them to belong mainly to the hundred years between 1580 and 1680, and