"I will now tell you," he continued, "about the structure of these masses of iron that fall from the sky. Their surfaces are more or less dotted and indented, and sometimes they suggest that there were large bubbles of air on them at the time they were cooled. One that fell near Lockport, in the State of New York, is honey-combed as though cavities had been dug in it with a chisel, and another that came from Santa Rosa, in New Grenada, has a less number of dimples in it, but they are deeper. And let me say, before I forget it, that this aerolite from Santa Rosa weighs 1600 pounds, and has about one cubic foot of volume; it is consequently very dense, and is composed of nearly pure iron." Fred asked if that was the largest meteor ever found. [Illustration: MELBOURNE AEROLITE.] "Not by any means," the Doctor answered. "The largest in any collection was sent from Melbourne, Australia, to the British Museum, and weighs 8257 pounds. The same museum has one from Otumpa, South America, that weighs 1400 pounds, and was detached from a mass estimated to weigh more than fifteen tons. This is the largest mass of meteoric iron known to exist. [Illustration: STRUCTURE OF THE TEXAS AEROLITE.] "Meteoric iron has a highly crystalline formation, which can be found by polishing a surface and then heating it to whiteness. When it is cooled it will be found covered with curious lines and streaks that remind you of the frost-marks on a window-pane. An aerolite found in Texas was submitted to the heating process; the streaks that were developed remained there permanently, and the same is the case with several others. Ordinary iron will not exhibit these marks, but they can sometimes be produced in iron that has been melted out of volcanic rocks." "How does the iron get up in the atmosphere to form these aerolites?" one of the boys inquired. "That is a conundrum I give up without trying," the Doctor replied. "Nobody has yet been able to tell us, and we must be content with the fact that it is there. "And there is a good deal of it up in the regions beyond the clouds," he continued, "if we are to judge by the number of meteors or shooting-stars that are seen every year. It has been estimated that more than a thousand meteors fall daily through the air so near the earth that they might be seen from one place if the clouds and the sun and moon would permit; taking this as a basis, there are more than 8,000,000 of meteors visible every day from all over the earth. Once in a while we have meteoric showers, when thousands of meteors can be seen from one point in an hour's time, and frequently the fall is so rapid that they cannot be counted. But it is a curious circumstance that in these showers no meteor or its fragments have been known to strike the earth."