312 ATMOSPHERE IN RELATION TO HUMAN LIFE AND HEALTH. 



appearance of the aurora greatly resembles the passage of voltaic elec- 

 tricity through Geissler's exhausted tubes. 



Observation is much needed in relation to these matters. The aurora, 

 from a meteorological point of view, is interesting as a proof of the 

 great height to which the atmosphere extends. Estimates of the height 

 of the phenomenon exceeding 100 miles have, however, not been fully 

 verified. 



METEORS AND AEROLITES. 



Meteors, or shooting stars, are within the domain common to both 

 astronomy and meteorology. Tbe moment they enter the atmosphere 

 they are objects of special interest to the meteorologist. It is known 

 that they traverse the air, where it is dense enough to raise them to a 

 white heat, at very great velocities. 



Many calculations have been made of the height of particular meteors 

 which have been observed over a wide stretch of country. The state- 

 ment by one astronomer many years ago as to the enormous numbers 1 

 which enter the atmosphere daily has been repeated so often, without 

 confirmation by the actual observation of others, that it would be well 

 to obtain independent values for particular areas on which to base 

 fresh estimates. The majority of shooting stars are probably telescopic 

 objects and of very small dimensions, perhaps not larger than pebbles. 

 Particles weighing only a few grains become visible to the naked eye 

 if they enter the air at a velocity of 40 miles a second. Many nights 

 pass in which, with a clear sky, only a very few shooting stars cross the 

 field of view. 



It has been suggested by a distinguished astronomer that meteors or 

 aerolites are the products of terrestrial or lunar volcanoes, which have 

 been shot out to so great a height that they escaped from the retaining 

 power of the earth's gravitation. In remote ages the density of the 

 air and the amount of vapor, and consequently the friction, must have 

 been greater than at present; but meteorology offers no objection to 

 the theory, and the problem of their terrestrial or extraterrestrial 

 origin is rather one for geology to assist in elucidating. 



ATMOSPHERIC TIDES. 



There can be no doubt that large tidal effects are produced in the 

 atmosphere by the sun and moon, but they are not easily detected, for 

 the barometer only registers the weight of the air and not the height, 

 and the weight of a column of certain height is diminished under the 

 crest of a tidal wave. Practically, however, solar and lunar gravita- 

 tion and their atmospheric tides have no important influence on weather. 

 Provisionally, the barometric effect of the lunar tide has been calculated 

 from observation to be from 0.003 to 0.004 inch. The interest of 

 the question lies rather in its astronomical bearing. The range or 



^our hundred million has been given by one computation. 



