“I mean that it was an extremely apposite accident for illustrating the subject upon which we were engaged at the moment of interruption.” “This is the second accident then,” observed Louisa, “that will have served us in our scientific studies. What a philosopher,” continued she, “must Dr. Doseall become, if he profit by every accident he witnesses!” “Knowledge, my dear girl, is not promoted by the opportunity of seeing, but by the faculty of skilfully observing, and reflecting upon what we see; were it otherwise, the merit of a traveller might be at once estimated by the number of shoes he had worn out. Whenever, therefore, you hear of a discovery having been made _by accident_, do not, on that account, depreciate the merits of its author. It is certainly true, that many an important truth has been brought to light from some casual observation(29), but the dexterity with which such observation was applied constitutes the merit of the discoverer. Well, but to show in what manner the accident of Tom Plank bears upon the subject under discussion:--He had ignorantly fired a quantity of oxygen and hydrogen gases in a tin vessel; the consequence of the combustion was the immediate formation of a _vacuum_: and what happened? Why, the pressure of the external air, not being any longer balanced by elastic matter in the interior of the apparatus, crushed it with violence, as any other enormous weight might have done; and so ended the accident, which report magnified into a most awful catastrophe.” As the party proceeded on their way home, they continued to discourse on the subject of the air’s pressure. “If the atmosphere exerts so enormous a pressure, and has so much weight,” observed Louisa, “it is strange that it should not fall down on the earth.” Mr. Seymour replied, “that the air was a peculiar fluid, which, from its elastic properties, was distinguished by the term of an _elastic fluid_, the particles of which were too far distant from each other to exert any cohesive attraction amongst themselves.” “But I suppose,” said Tom, “that it gravitates, or is attracted by the earth; what then can be the reason, as Louisa says, that it does not fall, like any other body, to the ground?” “And so it actually does,” replied Mr. Seymour. “The lower stratum of the atmosphere rests upon the ground, but the strata above it do not fall, because they are supported by the particles beneath them, in the same manner as the water at the surface of a basin is supported by that at the bottom: the only difference in these two cases arises from the one being an elastic, and the other an inelastic fluid; so that the air after compression resumes its original dimensions; and since the atmosphere, by the action of gravity, is always in a state of compression, so is it always endeavouring to expand itself.”