It is interesting to observe the grogress of the lightning rod in the favor of men competent to judge of its merits. Years ago it was the bane of the land, and the lightning-rod agent was as little favored as the hobo of today. Very likely there was a good reason for this, for he certainly was the Baron Munchausen of applied electricity, yet it is apparent that some of the things that he did without other reason than to sell plenty of his twisted rods are now looked upon with favor by those who have studied the protection of structures from lightning. The disfavor of the lightning rod is certainly diminishing, and recently the National Fire Protection Association in this country and the Lightning Research Committee in Great Britain have been paying much attention to the subject.

Fortunately, it is now possible to understand why destruction formerly occurred in structures well equipped with rods properly installed. These cases, which were largely the cause of the lack of confidence shown in such protection, we now know to be due to an unusual form of lightning, and on account of the general growth of interest in the subject, it is timely mention the new theory of these discharges. Some years ago it was pointed out by Sir Oliver Lodge and others, that lightning was of two classes, which that physicist named the A and B flashes, respectively. The A flash, as explained in the recent report of the Lightning Research Committee, is of the simple type that arises when an electrically-charged cloud approaches the surface of the earth without an intermediate cloud intervening. Under these conditions the ordinary type of lightning conductor acts in two ways ; first, by silent discharge, and second, by absorbing the energy of a disruptive discharge. In the second class, B, where another cloud intervenes between the cloud carrying the primary charge and the earth, the two clouds practically form a condenser. When a discharge from the first takes place into the second, the free charge on the earth side of the lower cloud is suddenly relieved, and the disruptive discharge from the latter to the earth takes such an erratic course that no series of lightning conductors of the hitherto recognized type suffice to protect the building.

The reason for this latter statement can be best explained by pointing out the change in the opinions held concerning the character of lightning discharges. Formerly, as Dr. Lodge recently pointed out, electricity was treated as if it had no inertia and all that was necessary was to get it from the clouds to earth as quickly and easily as possible by the shortest path. It is now recognized, however, that it is not so much the quantity of electricity that has to be attended to as the electrical energy. The problem is to dissipate as quietly as possible this electrical energy stored between the clouds and the earth in dangerous amounts.

A sudden dissipation of energy is always violent, and nobody in his senses tries to stop a heavy flywheel or a railway train suddenly. It is exactly the same with the store of energy beneath an electrified cloud or between one cloud and another. A perfect conductor, if struck, would deal with the energy in such a rapid manner that the result would be equivalent to an explosion. Hence the conductors must be of moderately high resistance. In any case, however, the rush is likely to be rather violent, and, like an avalanche, it will not take the easiest path provided for it. No one path artificially provided can be said to protect others, and the only safe protection is the impracticable one of encasing the building wholly in metal.

This statement explains why the points of conductors are sometimes inoperative in protecting a structure. When the energy is stored between cloud and cloud, instead of between cloud and earth, and the initial discharge is from one cloud to another, the lower cloud is liable to overflow suddenly to the earth through a region where there has been no previous preparation, and where any number of points or a rain-shower or any other means for a gentle leak would be quite inoperative. A violent discharge can then occur at the sharpest point, and in this connection it should be observed that a column of heated air like that rising in a chimney is even preferred by the lightning to an ordinary conductor. These considerations have led the British committee to make the following practical suggestions concerning protection against lightning: Two main lightning rods, one on each side, should be provided, extending from the top of each tower, spire or high chimney by the most direct course to earth. Horizontal conductors should connect all the vertical rods (a) along the ridge or any other suitable position on the roof, and (b) at or near the ground line. The upper horizontal conductor should be fitted with aigrettes or points at intervals of 26 or 30 feet. Short vertical rods should be erected along minor pinnacles and connected with the upper horizontal conductors. All roof metals, such as finals, ridging, rainwater and ventilating pipes, metal cowls, lead flashing, gutters and the like, should be connected to the horizontal conductors. All large masses of metal in the building should be connected to earth either directly or indirectly by means of the lower horizontal conductor. Where roofs are partially or wholly metal-lined, they should be connected to earth by means of vertical rods at several points. Gas pipes should be kept as faraway as possible from the positions occupied by lightning conductors, and as an addittonal protection the service mains to the gas meter should have a metallic connection with house service leading from the meter.

These suggestions are much less minute in their details than those recently made by the committee of the National Fire Proteciion Association, but it is not unlikely that they are about as useful in actual practice. They omit one suggestion that is strongly urged in this country by several investigators of lightning phenomena, and that is the provision of points and conductors on large trees in the vicinity of buildings. This provision, which seems to have originated in a suggestion made by Prof. Elihu Thompson, is a very good one for some cases. The most important feature of the work being done on both sides of the Atlantic is, however, the careful investsgation of cases of damage by lightning and the examination by men of high standing among electrical engineers of the data tbus acquired. This work will eventually result in lifting the ban imposed by the public on the lightning rod, and will add somewhat to the safety of buildings and high chimneys against a class of accidents which are commonly classed as unpreventable. - " The Engineering Record. "