MEASUREMENT OF EARTHQUAKE-WAVES. 589 



tncity of the atmosphere at the point where the disk is fixed affects 

 the electroscope and electrometer. Prof. Palmieri prefers the con- 

 ductor 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 nega- 

 tive, 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 power- 

 ful 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 con- 

 densed, 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 in- 

 formation, and it is much to be regretted that equally efficient ob- 

 servatories 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 Sci- 

 entific Inquiry ; " but there ought to be three or four as delicate as 



