Lightning rods neither prevent lightning stroke nor do they, as is sometimes alleged, attract lightning to buildings. They merely provide good conductors along which a stroke of lightning may reach the earth without doing damage, and, within very moderate limits, determine the path of discharge. While there are many unsettled points regarding the theory of lightning rods and details of construction, their general utility is strikingly indicated by statistics showing the comparative amount of damage done by lightning to rodded and unrodded buildings. According to the United States Bureau of Standards, information gathered in this country shows that “taking rods as they come in the general run of installations, they reduce the fire hazard from lightning by 80 to 90 per cent in the case of houses, and by as much as 99 per cent in the case of barns.” The same bureau, in its valuable publication, “Protection of Life and Property Against Lightning” (Washington, 1915), supplies the answers to a multitude of questions that are constantly asked about lightning rods. Buildings with metal roofs and frames connected with the ground are generally well protected from lightning (except as to nonmetallic chimneys) without rods. During actual thunderstorms, and also at other times when there are high potential gradients in the atmosphere, luminous electric discharges of a more or less continuous character are sometimes observed to occur in the shape of small jets and flames, chiefly from pointed objects, including lightning rods, the masts and spars of vessels, the angles of roofs, etc. These are identical in character with the “brush” discharges, or incomplete sparks, produced by electric machines. They are accompanied by a hissing or crackling sound. Their luminosity is comparatively feeble, and for this reason they are much more often observed by night than by day. They are especially common during snowstorms. This phenomenon is known as _St. Elmo’s fire_ or _corposants_ (not to mention a score of other names, ancient and modern). As seen at sea, corposants sometimes take the form of one or two starlike objects at the trucks of the masts or the ends of yard arms, but occasionally the spars, rigging and other parts of the ship are lighted up with a great number of stationary or moving flames, producing a weird spectacle. The finest examples of corposants are, however, observed on high mountains, and the phenomenon has been carefully studied at certain mountain observatories, such as those on Ben Nevis and the Sonnblick.