Popular Science Monthly/Volume 2/December 1872/Great Fires and Rain-Storms< Popular Science Monthly | Volume 2 | December 1872
|GREAT FIRES AND RAIN-STORMS.|
ASSISTANT PROFESSOR OF PHYSICS 1ST HARVARD COLLEGE.
THE belief that great fires are followed invariably by rain-storms is wide-spread, and the great fires of the present year in America, it is claimed, afford no exception to the law. The attitude of scientific men in regard to so-called popular fallacies and superstitions is not, in general, a praiseworthy one. A belief needs often only to be widespread among the people at large to be denounced. Science is but another word for truth, and even popular traditions deserve to be examined with care. The difficulties, however, in the way of an investigation of the effects of fires in producing rain-storms are manifold. Our knowledge of the science of meteorology is, at the best, very imperfect; and we have no series of observations from which we can draw trustworthy conclusions. A careful search into the narratives of great fires and into the accounts of great naval and land fights gives nothing which a scientific man would accept for a moment. One who is ready and determined to believe, it is true, will find in history many curious and apparent corroborations of the truth of his belief. Thus in Pepys's "Diary" there is a quaint and circumstantial account of the great fire in London. In speaking of the progress of the fire, he says: "So as we were forced to begin to pack up our own goods, and prepare for their removal; and did by moonshine (it being brave dry and moonshine and warm weather) carry much of my goods into the garden;" and in another place: "But Lord! what a sad sight it was by moonlight to see the whole city almost on fire that you might see it plain at Woolwich, as if you were by it." In still another place, in speaking of the poor sufferers made homeless by the fire: "A great blessing it is to them that it is fair weather for them to keep abroad night and day." He thus concludes: "Sunday.—So to my office, there to write down my journall, and take leave of my brother, whom I send back this afternoon, though raining; which it hath not done a good while before."
After reading this, we turn to the account of the burning of Moscow. But this occurred in September, and the equinoctial gales were blowing fiercely at the time. If we look into the history of land and sea fights, we find many striking and apparent confirmations of the truth of the popular belief. Froude concludes his description of the fight at Flores, 1591, as follows: "Nor did the matter end without a sequel awful as itself." Sea-battles have been often followed by storms, and without a miracle; but with a miracle, as the Spaniards and the English alike believed, or without one, as we moderns would prefer believing. "There ensued on this action a tempest so terrible as was never seen or heard the like before." The human mind is undoubtedly prone to connect great calamities together, and to believe that the one following depended in some mysterious way upon the one preceding.
We turn now to the great fire at Chicago. It was telegraphed to London, England, that "this fire was chiefly checked on the third or fourth day by the heavy and continuous down-pour of rain, which, it is conjectured, was partly due to the great atmospheric disturbances which such an extensive fire would cause, especially when we are told that the season just previous to the outbreak of the fire had been particularly dry." In an article published in the "Journal of the Franklin Institute," July, 1872, by Prof. I. A. Lapham, assistant to the Chief-Signal Officer U. S. A., entitled "The Great Fires of 1871 in the Northwest," we find the following in regard to the burning of Chicago: "During all this time—twenty-four hours of continuous conflagration upon the largest scale—no rain was seen to fall, nor did any rain fall until four o'clock the next morning; and this was not a very considerable 'down-pour,' but only a gentle rain, that extended over a large district of country, differing in no respect from the usual rains. The quantity, as reported by meteorological observers at various points, was only a few hundredths of an inch. It was not until four days afterward that any thing like a heavy rain occurred. It is therefore quite certain that this case cannot be referred to as an example of the production of rain by a great fire. Must we therefore conclude," says Prof. Lapham, "that fires do not produce rain, and that Prof. Espy was mistaken in his theory on that subject? By consulting his reports (Fourth Report, 1857, p. 29), it will be found that he only claimed that fires would produce rain under favorable circumstances of high dewpoint, and a calm atmosphere. Both of these important conditions were wanting at Chicago, where the air was almost entirely destitute of moisture, and the wind was blowing a gale. To produce rain, the air must ascend until it becomes cool enough to condense the moisture, which then falls in the form of rain. But here the heated air could not ascend very far, being forced off in nearly an horizontal direction by the great power of the wind. The case therefore neither confirms nor disproves the Espian theory, and we may still believe the well-authenticated cases where, under favorable circumstances of very moist air and absence of wind, rain has been produced by large fires." Prof. Lapham also remarks, "The telegraph-wires indicated no unusual disturbance of the electrical condition of the atmosphere." Upon reading this last remark, the question occurs to us, Can there not be a change in the electrical state of the atmosphere which, although too small to manifest itself upon telegraph-wires, may occasion storms?
Some experiments made by the writer in the physical laboratory of Harvard College, on the influence of flames upon the electrical state of the air, may throw some light upon this subject. Two pieces of apparatus were used, one of them "the new quadrant electrometer" of Sir William Thomson, and the other a "water-dropper," also an invention of that distinguished philosopher. The electrometer is a very complicated piece of apparatus. Let me describe it in as clear a manner as possible. I do not know that I shall succeed in conveying to the uninitiated any idea of that instrument, for it has many parts. I shall endeavor merely to explain its principles roughly. Conceive of a light aluminum needle, suspended by two single cocoon threads in the centre of a glass jar, which is filled to nearly one-sixth of its capacity with strong sulphuric acid. A very fine platinum thread drops from the aluminum needle and dips in the acid. Let us see what we have now. An aluminum needle suspended in mid-air by two filaments of silk very near each other, and so fine that they can hardly be perceived by the naked eye. Further, this needle has an extremely fine metallic wire running down from it and terminating in a little weight, also of the metal platinum, which is immersed in the sulphuric acid. Thus we see that the needle is very free to swing in a horizontal plane, and it will be readily perceived that, if there were but one filament of silk supporting it, it might swing round a complete circumference, or indeed make many revolutions under the influence of a strong repellant or attractive force; the two filaments by their torsion allow the needle to swing only to a certain distance, and compel it to return to its original position when the force is removed. One can readily conceive of this by suspending a bar in an horizontal position by two vertical ropes, and then endeavoring to turn it in an horizontal plane.
Let us now charge the aluminum needle with positive electricity. To do this, we shall conduct into the sulphuric acid by means of a metallic wire a slight positive charge, and the acid, being a good conductor, will convey this charge by the extremely fine metallic wire to the aluminum needle suspended above the acid in mid-air. Now, if we present a substance charged with negative electricity to the needle, it will, as is well known, be speedily attracted; and if the substance presented has a positive charge it will be repelled: the unlike charges attracting and the like repelling each other. Thus, we see that we have a very delicate test for the character of the electricity in the body which we bring into the neighborhood of the needle. But, the needle being only about two inches in length, slight movements in it can hardly be detected. How is this to be remedied? We are going to deal with delicate impulses, and it is necessary to have some means of observing them. Standing at this window, through which the sun streams brightly, with a little mirror, we can, as any school-boy knows, throw the image of the sun in almost any direction that we please. Now it rests upon the brick wall across the street, a hundred feet or more distant from us. Let us turn the glass slightly: see how small a movement suffices to make the sun's image on the wall dart over at least twenty feet! Why can we not attach a little mirror, which shall not weigh more than a feather, just above our needle, and let it reflect, instead of the sun, a little point of light from a kerosene-lamp upon yonder wall which is four feet from it? We have done so. We allow the light of the lamp to stream through a small opening in a screen of blackened paper, and to fall upon the mirror. Upon presenting this metallic plate, which has been charged with positive electricity, the spot of light darts along the scale pasted on the wall; it has gone over nearly six inches of the scale, while the motion of the needle and the mirror was hardly perceptible. We have now an extremely delicate test for the presence of electricity—so delicate that even the small charge ever present in our bodies is sufficient when we approach the instrument to make the spot of light dart to and fro. It is only necessary now to have some convenient means of presenting the body to be examined to the needle; for it will be seen that all movements of the air in its neighborhood must be avoided. To accomplish this end, we surround the needle with four plates of brass, which are carefully separated from the needle. They are in the form of sectors of a circle and lie in an horizontal plane, the suspension fibres of the needle going through a round hole in the centre of the circle of which the sectors form a part. These sectors are separated from each other at first; the opposite pairs can, however, be connected at will. It is not necessary to dwell upon their peculiar construction: their object is to prevent the charge, led to them by these copper wires, running to any part of the room, and thereby to influence the needle. It will be seen that this instrument, of which we have explained only the principal features, is wonderfully delicate, and far superior to the old electrometers which showed electrical attraction and repulsion by the divergence of two suspended gold leaves.
It remains now to describe the "water-dropper." This consists merely of a tin vessel carefully insulated at the base, with a long glass tube projecting from an orifice near the bottom. The water runs through this tube and issues in a fine stream from its end, breaking into drops about fourteen inches below it. A collecting-plate connected with one of the brass plates which we have described in the electrometer stands under this stream of water. Now the drops of water in their fall upon the plate remove by their impact the charge which the plate has by itself—for all bodies have a greater or less electrical condition, and the plate then takes the electrical condition of the air in which it is immersed.
Let us place our water-dropper on the window-sill with its tube projecting into the open air; and, having placed the collecting-plate so that the drops of water may strike upon it, let us notice our little spot of light. It is a clear day in early summer; there are no clouds to be seen, save a rift away on the horizon in the west. The spot of light moves gradually over the scale, indicating that there is a slight positive charge of electricity in the atmosphere. Now it is stationary, and we are about to record the reading of the scale, when the spot of light gives a quick jump and then returns to nearly its original position. Perhaps some movement of ours has deranged the instrument. We look at it carefully, and return to our position of observation. A low rumble, as if of distant thunder, is heard. We do not mind this at first; presently the spot of light darts again along the scale, and again returns to its original position. We stand in silence, waiting for further developments. Now, we hear again a rumble, and a low muttering, as if of thunder in the west. Can this movement of the spot of light have any connection with the distant lightning? At least five minutes must have elapsed between the time of the movement of the spot of light and the moment when the thunder was heard. Again the spot moves, again follows a low peal of thunder; again and again the same phenomenon is observed. There can be no doubt of it: the electrical discharges of the approaching storm, yet miles away, are registered by this little instrument in our laboratory. Now the storm approaches nearer. We hear the wind in the trees; a few drops fall upon the tinned roof; the lightning darts hither and thither, and the spot of light leaps responsively to it.
Such, then, is the delicacy of our instrument. By allowing the spot of light to fall upon sensitive paper, which moves along by clock-work, we shall have all of its motions recorded by photography. This registration has been accomplished by Sir William Thomson, to whom we owe so many beautiful electrical instruments. It would be well if our Signal Service should make contemporaneous observations in different parts of the country by means of these instruments. Such observations could not fail to throw light upon the connection of electrical storms with rain-storms, and extend our knowledge of meteorology.
Let us now, having proved our instruments, approach the question of the influence of flames upon the electrical state of the atmosphere. Our observations must be made in the laboratory, and are necessarily of a somewhat general character from the nature of the subject. It is a cold, clear day in early winter, with the wind blowing freshly from the northwest. "We know by various indications that the air is highly electrified. The wind blows too freshly to place our water-dropper out-of-doors; accordingly, it is placed upon a table in the laboratory, and we notice the indication of the instrument. The air is indeed highly charged—the spot of light is thrown to a greater distance upon the scale than we have often noticed. Let us record the reading; it is as much as the positive pole of twelve Daniell cells gives. Now, lighting our Bunsen gas-burner, and placing it near our water-dropper, let us observe the spot of light; it does not change its position materially at first.
Now, after the lapse of a few minutes, it falls to a lower position upon the scale, now it goes down to zero. Now it mounts again, but in a contrary direction to its first indication, showing a slight negative charge in the air, which before was strongly charged positively. Can this be due to the presence of the flame? And, if it is due to the flame, are not great fires capable of influencing the electrical state of the atmosphere, to a greater or less extent? Such are the questions which force themselves upon us. We must, in the first place, examine our flame. Soldering a platinum wire to the copper connecting wire, and dispensing with the water-dropper, we examine the outer and inner cone of the flame. At all points in the luminous portion of the flame an indication of negative electricity is obtained; at all points in the immediate neighborhood of the flame, but exterior to it, there are signs of a positive charge in the heated air. The inner cone of partly-consumed gas is neutral, or slightly negative.
The flame, therefore, is negative, and it tends by its presence to reduce the positive charge of electricity which generally characterizes the air of fine weather to zero, or to change it to a feebly negative-charge. We learn from various writers upon meteorology that the normal electricity of the atmosphere is positive. Herschel in his work on Meteorology says, p. 125: "Out of 10,500 observations made at the Kew Observatory in 1845–'47, 10,176 showed positive, and only 364 negative electricity—the latter being almost always accompanied with heavy rain." Sir William Thomson, in the proceedings of the Literary and Philosophical Society of Manchester, March, 1862, relates some experiments which tend to show that clearing-up weather is preceded, and in many cases foretold, by a change in the atmosphere from a negative to a positive charge of electricity.
We can, therefore, conclude with some probability of truth that great fires, by changing the electrical state of the atmosphere, have an influence upon the production of rain. The state of our knowledge, however, in regard to the part that electricity plays in atmospheric changes, is very meagre. The question of the truth of the popular belief that great fires are followed by rain still remains unanswered; and we can only hope that we have thrown a little more light upon it by our research.