The Numbers of Meteors.—It is hence impossible at present to give even an approximate estimate of how many meteor streams have so far been observed, more than to say that the number is large. Better data can be given as to hourly or monthly numbers of all meteors visible to the eye. It is found that nearly twice as many are seen per hour during the second half of the night as in the first half. Also meteors are twice as numerous from July to January as from January to July. The second phe nomenon is due simply to the earth meeting more meteor streams in that part of its orbit ; the first to the fact that in the evening hours the meteors must overtake the earth to become visible, while in the morning hours the meteors meet the earth head-on. Of necessity, therefore, we see more after midnight than before. From January to July, for the whole night, an hourly average of six or seven should be seen by one observer with a clear, moonless , sky; from July to January an average of 12 to 15. Excluding telescopic meteors, of which there must be tens of millions, it is estimated that zo million meteors enter our atmosphere daily, and a minimum of one meteorite per day reaches the earth's surface.
Fireballs and Bolides.—These bodies, which are merely super-meteors, owing to their larger mass, are able to penetrate lower before destruction. Also when they overtake the earth before midnight with low velocity, they sometimes have paths many hundred miles long in our atmosphere, which often do not make large angles with its surface. These circumstances permit them a relatively longer survival. Such bodies may give every variety of phenomenon mentioned above. When they come quite near the earth's surface their passage is frequently accompanied by very loud sounds, which are sometimes said to be like explosions but more properly may be compared to the "shock-wave" of a great projectile. Many compare them also to distant thunder. Such fireballs of very long paths may actually cross from horizon to horizon for one observer and pass far beyond.
The most remarkable example of this type of phenomenon is furnished by the "Meteoric Procession" of Feb. 9, 1913. This wonderful group of bright meteors or fireballs was first seen over Canada, and having travelled about 5,700 miles was last seen over the Atlantic, but still going towards the south-east. It con sisted of four or five groups of 4o to 6o members each. Along the observed part of their path their height was only about 35 miles. At many of the Canadian stations their passage produced sounds like thunder, and at eight stations even houses were shaken. A similar phenomenon was seen at Cairo, in 1029 A.D., although unhappily the record is brief.
The brightness of most fireballs will vary greatly, generally increasing towards the end of their path. Minor explosions are often seen before the great final outburst that ends the career of so many of them. It is not unusual for such objects to have an apparent diameter equal to that of the moon, and to give about as much light as that body. Occasionally one has been reported
as making the night as bright as day, but we may well consider this an exaggeration. As such bodies are never expected, all observers suffer the serious disadvantage of surprise.
Heights and Velocities.—The heights of meteors in the at mosphere may be determined by observation and calculation, following well-known rules. The same object must have the co ordination of its end points observed from at least two places whose distance apart is known. How a meteor is actually ob served is described later. With the data mentioned it is relatively a simple matter to calculate the heights at which it began and ended as well as its length of path. During the last century and a quarter this has been done for many hundred such objects, varying from the smallest meteor observable to the naked eye to the largest fireballs. The following table gives the average results: If it is asked why the Leonids appear at higher altitudes than the Perseids, it is due to the fact that they strike the earth more nearly head-on. This gives them a greater relative velocity and hence they can begin to glow in a less dense part of our atmos phere.
Luminosity, Mass and Size.—It is obviously impossible that dust, gas or solid particles could continue to glow, merely from having been heated, in the intensely cold upper air for periods of many minutes. The part of a meteor's path that seems to give long enduring trains is from 6o to 5om. above the earth's sur face. Study with the telescope has shown them to be tubular in shape. Suggested explanations are that the trains are due to phosphorescence, possibly connected with electric discharges; or that part of the energy being stored up in the molecules is later released as a gradual emission of light.
All reliable calculations of the masses and diameters of me teors lead to surprisingly small values. Meteors of the first and second magnitude are thought to be not more than one- or two tenths of an inch in diameter, nor weigh more than a few milli grams. All such bodies undoubtedly appear larger owing to the envelope of heated and glowing gas that surrounds them in their flight, as well as to irradiation. There appears no sound reason for thinking small meteors different in constitution from their larger brothers, the meteorites. Analyses of the latter give us the usual elements, iron and nickel being very prominent in the me tallic ones. But many are wholly composed of stone. We may safely infer that smaller meteors have a similar constitution and are merely smaller fragments. That different elements predomi nate in individual meteors is further proved by their different colours. In a few cases the spectra of meteors have been photo graphed. These spectra also show familiar elements, such as hydrogen, calcium, magnesium, carbon, helium and sodium, but this analysis gives no conclusive evidence as to their source.