Page:Popular Science Monthly Volume 1.djvu/605

Rh of the atmosphere at the point where the disk is fixed affects the electroscope and electrometer. Prof. Palmieri prefers the conductor above described, to a conducting point or a flame, because he considers that these do not give comparable results, an objection which is not supported by all observers. He considers the same to be true of the method of dropping water.

After having made careful observations on atmospheric electricity for about a quarter of a century in a country where meteorological changes are more regular and less capricious than in our own island, there is no one whose deductions are more deserving of our attention; the more so as he considers that he has combined his researches into a definite law. His first fact is this: If within a distance of about fifty miles there is no shower of rain, hail, or snow, the electricity is always positive. The single exception is during the projection of ashes from the crater of Vesuvius. During a shower he finds the following: law universally to hold good: At the place of the shower there is a strong development of positive electricity; round this there is a zone of negative, and beyond this again positive. The nature of the electricity observed depends upon the position of the observer with respect to the shower, and the phenomena will change according to the direction in which the shower is moving. Sometimes negative electricity may be observed during a shower; but this is always due to a more powerful shower farther off. These conclusions have been supported by means of telegraphic communication with neighboring districts. It appears, then, that except when the moisture of the air is being condensed, there is no unusual development of electricity. These, results are in accordance with the experiments of Palmieri and others, which show that aqueous vapor in condensing develops positive electricity. No unusual development of electricity has ever been detected by him in a cloud when no rain is falling.

The above results, though falling short of what has to be done to complete the theory, are yet definite, and hence valuable, the more so if supported by other observers placed in equally favorable situations. But of the variations in intensity of positive or negative electricity nothing has been said.

Besides the fixed instruments at the Observatory, others are used on the mountain. Gases are collected from cracks in the earth's crust, tubes being let down into them, and the gas sucked up by a kind of bellows, to be examined at leisure. A portable spectroscope is also used during eruptions, and there is a larger one by Hoffman in the Observatory. From this Observatory we have received valuable information, and it is much to be regretted that equally efficient observatories have not been established in different parts of the world. Many portable and cheap instruments have been invented, most of which are described by Mr. Mallet, in the "Admiralty Manual of Scientific Inquiry;" but there ought to be three or four as delicate as