Popular Science Monthly/Volume 20/February 1882/Extension of the Signal Service
By Professor JOHN TROWBRIDGE.
THE Signal Service of the United States performs a great service to science by inculcating the value of scientific investigations. Observations in meteorology were formerly buried in the proceedings of learned societies, and met the eyes only of a few scientific men: now they are discussed at the breakfast-tables of a million people. The departments of physics in our universities and technical schools have reason to thank the Signal Service for leading the public mind to see the importance of physical investigations to the ordinary pursuits of mankind. He, however, who endeavors to raise money at the present time to equip a physical laboratory in connection with a university, will find that the Signal Service still has work to do among the practical business-men of this country—men whose successful prosecution of their commercial enterprises depends upon the economical use of steam, upon careful scientific consideration of atmospheric changes, conditions of heat and moisture, and sanitary laws. There are men of this class who accept what they call progress, without inquiring into the causes of it, and who have never thought of physics as a science to which business is indebted for its successful prosecution. "What is meant by the word physics?" asks the man who consults the paper to discover if the storm-center which has been developed in Texas, and whose progress has been telegraphed, will probably strike his steamship, which is off Cape Hatteras. This steamship has recently been fitted up with the most approved compound engines, which have been perfected by the labors of those physicists, Rankine and Thomson. "What is meant by the word physics?" asks the manufacturer, with a telephone to his ear, listening to a response from his factory. The solicitor for the endowment of a physical laboratory has the satisfaction, even if he fails to obtain a subscription, of feeling that he is a public educator.
It is very probable that any plan for the extension of the Signal Service would meet with opposition, even among those who would be most benefited by such extension. Yet the demands of science are daily becoming more inexorable, and the progress of meteorology toward the stand-point of an exact science is assured by the labors of those who have charge of the Signal Service.
At one time most of our American colleges and universities conducted meteorological observations. These observations were industriously made and conscientiously carried on, but were very much like private measurements of rain-fall and barometer heights—notes of the first coming of the golden robin in spring—and journals of cold or hot days. Meteorological observations which are not taken simultaneously over extended areas are of little value, save to the compilers of local almanacs. The Signal Service, having relieved four universities of their onerous meteorological responsibilities, doubtless feels indebted to them for pointing the way, and would be grateful if the universities could lead them one step higher, since it is one of the functions of a university to be always a little in advance of the ruling conditions of popular knowledge. Some plan of co-operation might be devised, by which the universities and colleges along our Atlantic sea-coast could aid the Signal Service in testing the value of more delicate investigations upon atmospheric changes than can be carried on at the present Government stations. Harvard University, Yale, Columbia, Princeton Colleges, the University of Pennsylvania, and Johns Hopkins University could, doubtless, provide suitable rooms and experienced assistants for testing the value of simultaneous observations upon various phenomena which are not at present taken into account in weather predictions. We judge at present of climatic changes by observations aboveground; it may be that the presence of earth-currents of electricity, of fluctuations in the earth's magnetism, of waves of heat through the superficial layers of the earth, may reveal important factors in announcing meteorological changes. Simultaneous observations, upon the slight earthquake-shocks which are continually pulsating beneath the apparently calm surface upon which our great cities are built, may have important relations to conditions of heat and cold. The mere mention of these unobserved phenomena is sufficient to show the state of our ignorance, and to lead us to expect that investigations in physical laboratories will be of practical value in leading the Signal Service to extend its usefulness.
The electrical state of the air is supposed to have great influence upon the proper conditions for fair weather and for storms, and to also affect the states of health and disease; but no definite information has been collected which bears upon these points. The Signal Service would aid the science of meteorology very greatly by extending observations on the electrical state of the air over large areas of territory.
The apparatus for studying the electricity of the atmosphere is probably the most delicate and refined of any in the subject of electricity. Sir William Thomson's electrometer is an admirable instrument, wonderful in its details and in its scientific plan. It can be used, however, by any intelligent observer. In principle it consists merely of a suspended aluminum needle which swings between four hollow quadrants. The needle is put in constant contact with the inner coating—which consists of sulphuric acid—of a Leyden-jar, by means of a small wire suspended from the needle. The diagonal quadrants are connected. If one pair is put in connection with the ground and the other pair connected with the body whose electrical state is to be tested, the aluminum needle by its deflection indicates the kind and amount of the electrical state. The needle is provided with a mirror, which allows its deflections to be observed by means of the reflection of a point of light. Although the principle is simple, yet the means for obtaining complete insulation and for keeping a constant charge in the needle are quite complicated. The Leyden-jar can be charged by means of a small Holtz machine, or by means of an induction-coil. A still better method of charging has been adopted by the Continental physicists: this consists in the employment of a miniature water-battery of five hundred or a thousand cells. Small glass vials, similar to those used by the homœopathists, are filled with distilled water, are placed in a wooden box, and are well insulated from each other. A sheet of zinc and copper are soldered together along their edges, and then cut into small couples of zinc and copper. These cells are coupled seriatim, or, as it is usually termed, for intensity. The entire battery need not occupy a space of more than a foot and a half square, and can be made even more compact. The statical charge at the terminals of this miniature battery is strong, and a Leyden-jar or a condenser can be easily charged by connecting one terminal of the battery to the inner coating of the condenser and the other to the outside coating. The electrometer-jar is charged, therefore, in this way, by being kept permanently connected with this battery.
The apparatus for collecting the charge of the air, in order to affect the charged needle, consists of an insulated tin can filled with water. The water is allowed to flow from this can by means of an horizontal glass tube, which is drawn out to a comparatively narrow orifice. The can is then connected by an insulated wire to one pair of the quadrants of the electrometer. Since the original charge of the can and the water—all objects are necessarily charged with electricity to a greater or less extent—is dissipated by the flow of the water, the electricity of the air continually renews this loss, and the tin can finally takes the charge of the air in which it is placed. This method is very sensitive, and will detect discharges of lightning by a throb of the electrometer-needle even when the thunder-storm has not yet appeared above the horizon. A method of obtaining continuous registrations of the deflections of the electrometer-needle is necessary. This can be accomplished by receiving a spot of light, which is reflected from the mirror attached to the needle, upon a cylinder covered with sensitized paper. This cylinder is made to slowly revolve by clock-work, so that twenty-four-hour observations can be obtained. Nothing is so characteristic of modern methods in physical science as continuous registration of physical phenomena. Early observers were forced to content themselves with scattered observations taken at different intervals, and important variations might occur at the times when no observer was watching the apparatus. Now the movements of the human heart can be recorded by a little apparatus which will combine in one curve a thousand continuous observations, and the slightest anomalous fluctuations can be studied from the record which is obtained.
Provided, therefore, with these modern and more refined means of observation, we are in a condition to study the fluctuations of that subtile manifestation of energy which is ever present in the air, and seems to lurk in all matter. The influence of the electrical state of the air upon atmospheric changes is doubtless far-reaching. We are accustomed to think of thunder-storms as the only manifestations of atmospheric electricity; but there are influences which it exerts, more silent than those which are announced by the crackle of lightning, yet none the less extended. A pretty experiment, described by Lord Rayleigh, illustrates the effect of electricity upon the coalescence of rain-drops. A narrow stream of water is allowed to issue from a reservoir and form a parabola, which strikes the ground in drops about four feet from the orifice. When this is a fine stream, it breaks up into a shower of drops at about two feet from the orifice. On rubbing a bit of sealing-wax with a cloth, and presenting the rod of sealing-wax near the stream, the drops immediately cease to separate, and the stream is continuous from the orifice to the point where it strikes the ground. This is a very striking experiment, and undoubtedly has a bearing upon the formation of large drops of rain, which are often noticed during a thunder-storm after a discharge of lightning. Instead of a rod of sealing-wax, a piece of writing-paper can be readily charged by placing it upon a table and rubbing it vigorously with the palm of the hand.
None of the theories which endeavor to explain the source of the electricity of the air are satisfactory. Professor Tait, in a recent lecture, is of the opinion that evaporation is the source. Under the influence of the sun's heat, this operation of nature is conducted on an immense scale, and seems to him to be competent to furnish the supply of atmospheric electricity.
It is maintained by others that the atmosphere of the earth has a permanent charge, which it received in the beginning, or which was developed by cosmical changes; and this charge manifests itself here and there, and fluctuates under different conditions of heat and moisture. Mr. Aiken has lately broached a novel theory of the formation of fogs and rain-clouds, which also has an important bearing upon the theory of atmospheric electricity. He believes that dust, or impalpable saline particles, are necessary for the formation of clouds. Without dust, or these particles, we should always have cloudless skies. To prove this theory, he caused water to evaporate in two receivers, the air in one of which had been passed through cotton-wool and other media, to intercept the dust; in the other the air contained dust. The usual fog which is perceived when water is suddenly evaporated under a receiver was absent in the receiver whose air had been deprived of its dust, but was present as usual in the other receiver. Various experiments of the same nature were tried. Fog suddenly formed in the dust-deprived air of a receiver if a bit of wood was burned, and thus caused to throw off small particles; burning sulphur was very active in producing these fogs. The fogs of London are thus partially caused by the consumption of coal and the evolution of sulphurous gas, and the fogs along the sea-coast by the saline dust formed from the spray.
If this theory is a correct one, our views of the electrical charges upon different layers of air must be modified to a considerable extent. Some experiments are now being made in the Physical Laboratory of Harvard University to test the inductive capacity of dust-films, and also to trace the fluctuations in the electric spark when it is caused to pass through air deprived of its dust, and air which contains impalpable matter. In order to add to our knowledge of the electrical conditions of the air, simultaneous observations are needed at a large number of stations. The cost of fitting up such stations would not fall short of six hundred dollars for each station, and the services of experienced observers would be necessary. It is not likely that the United States Government will establish such stations unless it can be shown that observations upon the electrical state of the air are important in predicting the path of storms. In the mean time, the various colleges in the different States might, by some system of co-operation with the Government, undertake such observations, since there are in nearly every college rooms and observers at command. As soon as it became apparent that the observations thus taken were of value, the interests of commerce would demand that they should become a part of the regular work of the Signal Service.