The work done at Blue Hill shows the importance of cloud observations to elucidate the general movements of the atmosphere, as well as the circulation of the air above barometric maxima and minima, which can result practically in making accurate weather forecasts possible a day or two in advance. The systematic observation of the upper currents was brought to the attention of the International Meteorological Committee by Dr. Hildebrandsson in 1885, and at the meeting of the International Cloud Committee in 1894, besides the adoption of the nomenclature of clouds and instructions for observing them, it was decided that observations of their motion, as well as measurements of their height, should be made in various parts of the world. Accordingly, the year commencing May 1, 1896, was designated as the "International Cloud-Year," and observations with nephoscopes of the direction of motion and relative velocity of clouds were begun at many stations in Europe and Asia, and at fifteen stations in the United States. Trigonometrical measures of the heights of clouds were undertaken at stations in Norway and Sweden, Russia, Finland, Prussia, and France, as well as at Toronto, Manila, and Batavia; in the United States the measurements already described were recommenced at Blue Hill, and the Weather Bureau equipped a similar station in Washington. In Europe it is thought that the determination of heights by photogrammeters, as the theodolites with attached photographic cameras are called, possesses advantages over the visual theodolites, and it is true that not only is the kind of cloud recorded on the plates, but there are available for calculation as many points on the cloud as can be identified on the two plates exposed simultaneously at both stations. On the other hand, in the case of nearly uniform or dark cloud-strata, it is easier to see points for measurement on the cloud than to fix them on the photographic plates. For this reason, and from the difficulty of manipulating the photogrammeter, visual instruments were adopted both at Blue Hill and at Washington. The work was successfully carried on until May 1, 1897, and the observations and measurements were reduced at Blue Hill according to the plan prescribed by the Committee. Already the observations and measurements made at Upsala, Manila, and Blue Hill are published, and the others will follow. The discussion of the correlated data from the various countries will probably increase our knowledge of the circulation of the atmosphere, which is certainly one of the most interesting and important questions in the physics of the globe. The result will have been reached by international co-operation, of which the benefits to science are everywhere manifest to-day. But for the whole problem to be solved, it is necessary, not only to know the movement of the air, but, as far as possible, to ascertain its conditions of heat and moisture. This may be accomplished by the use of balloons and kites, to be described in the remaining chapters.

CHAPTER III

BALLOONS—NOTABLE ASCENTS AND RESULTS OBTAINED—CAPTIVE BALLOONS

In the first chapter the invention of the hot-air and the hydrogen balloon was chronicled, and it was stated that on December 1, 1783, Charles rose from Paris to a height of 9000 feet. Public interest in France was greatly excited by this wonderful extension of the realm of man, and numerous ascensions with Montgolfières and Charlières, as the hot-air and hydrogen balloons were respectively called, took place in Paris and the provinces. The uses of the balloon seemed innumerable, and Lavoisier was instructed by the Academy of Sciences to draw up a report on the value of the new discovery. After having described in detail the ascensions which he had witnessed, the great chemist stopped, appalled at the multitude of problems which the balloon could solve. History has shown, however, that no commercial application of the balloon was possible, and that aside from its spectacular attractions, its chief use has been for scientific observations.

The first persons in England who devoted themselves to aërial navigation were foreigners. Two of them were Italians, the philosopher Tiberius Cavallo, who already in 1782 had showed to a London assembly that soap-bubbles filled with hydrogen will rise, and therefore had almost anticipated the invention of the hydrogen balloon, and the diplomatist Vincent Lunardi, who made some daring balloon ascents in 1784. But the honour of making the first scientific balloon voyage is due to a Bostonian, Dr. John Jeffries. Dr. Jeffries graduated at Harvard College in 1763 and then practised medicine in England, where he became a loyalist, and during the Revolution was with the British troops. In London he interested himself in aerostation, and, aided by the Royal Society, ascended in a balloon because, he said, "I wished to see the following points more clearly determined: first, the power of ascending or descending at pleasure, while suspended and floating in the air; secondly, the effect which oars or wings might be made to produce towards the purpose and in directing the course of the balloon; thirdly, the state and temperature of the atmosphere at different heights from the earth; and fourthly, by observing the varying course of the currents of air, or winds, at certain elevations, to throw some new light on the theory of winds in general." A French professional aeronaut named Blanchard had made three ascents in France and one in England, and Dr. Jeffries paid one hundred guineas to accompany Blanchard on his fifth ascent, which was made from London November 30, 1784. He took with him a thermometer, a barometer, a hygrometer, an electrometer, and a mariner's compass, also several numbered bottles, filled with water and provided with glass stoppers, which were to be emptied and corked up at different heights in the atmosphere. It was arranged to record the observations on ruled paper with a silver pen, because the doctor would not trust a common pen or pencil as liable to accident. He also had a map of England to determine the direction which the balloon took. Jeffries' English sentiments are shown by this quotation from his narrative: "I had provided a handsome British flag, invidiously represented the next day in one of the public papers to have been the flag of the American States." The barometer and thermometer were observed every few minutes, and the hygrometer occasionally. The electrometer did not change its indications. Samples of air were obtained and sent to the Royal Society, but it does not appear that they were ever analyzed. The balloon rose nearly two miles, and descended safely in Kent after an hour and a half. Jeffries' observations compare favourably with those made until recently; indeed, for nearly a century there was little improvement in the apparatus. The decrease of temperature which Jeffries found, viz. 1° for 360 feet rise, and the decreasing humidity with height agree very well with later observations.

Jeffries and Blanchard undertook a more perilous voyage on January 7, 1785, from Dover across the Channel, landing in the province of Artois, after, so runs the announcement, "we were suspended and floating in the atmosphere two hours over the sea and forty-seven minutes over the land of France." The voyagers were cordially welcomed, and were entertained lavishly in Paris as being, Jeffries says, "the first who passed across the sea from England into France by the route of the air." No instruments but a barometer and a compass were carried, and the only scientific result worthy of notice was that the balloon seemed to lose buoyancy over the sea, due to what Jeffries thought might be "the power of attraction over the water." The height of the balloon was measured trigonometrically by French officers in Calais, who found by angular measures, when the balloon was midway across the Channel, that its height was 4500 feet. Jeffries' voyages have been described somewhat at length because the first scientific balloon voyage is generally attributed to the Belgian physicist, Robertson, who ascended from Hamburg in 1803 to the improbable height of 24,000 feet. Robertson made his third ascent the next year from St. Petersburg, accompanied by the Academician Sacharoff. This was a scientific voyage, instituted at the request of the Russian Academy, to ascertain the physical state of the atmosphere and the component parts of it at different heights, also the difference between the results of vertical ascents and the observations of Deluc, De Saussure, von Humboldt and others on mountains, which it was rightly concluded could not be so free from terrestrial influences as those made in the open air. Among the experiments which the Academy proposed were the following: change of rate of evaporation of fluids, decrease or increase in the magnetic force, inclination of the magnetic needle, increase of heat of the solar rays, fainter colours in the spectrum, influence of rarefaction of the air on the human body, as well as some other chemical and philosophical experiments. A height of about two miles was reached, and many interesting observations were made, but since the instruments were not easily used in the basket of the balloon, the results were unsatisfactory and required repetition to be conclusive.

The Academy of Sciences of Paris now took up the investigation with the special object of proving whether the magnetic force decreased as Robertson in a balloon and De Saussure in the Alps had supposed. Two young physicists, Biot and Gay-Lussac, were chosen to carry out the investigations. They ascended from Paris on August 24, 1804, provided with all necessary instruments, but the balloon was too small to rise higher than 13,000 feet. Gay-Lussac ascended alone to a height of 23,000 feet on September 16, 1804, in a balloon filled with hydrogen. His observations confirmed those which he had made with Biot, that there was no change in the magnetic force, and from samples of air collected he proved that the chemical constitution of the air is invariable. His observations of temperature seemed to confirm the theory of a decline of temperature of 1° in 300 feet of elevation. The air was found to be very dry, and Gay-Lussac noticed that at the highest altitude the clouds were still far above him.

Passing over several notable ascents in other countries, it was not until 1850 that scientific ballooning was begun again in the land where the balloon originated. Then MM. Barral and Bixio made two ascents from Paris in rainy weather to the heights of 19,000 and 23,000 feet respectively, although they had expected to attain twice these altitudes. Their most interesting observations were the great thickness of the cloud mass, which in one case amounted to 15,000 feet, and the sudden fall of temperature in it from +15° to -39°. Some curious optical phenomena were connected with the floating ice crystals, and although the light of the sky was found to be strongly polarized, the light reflected from the clouds was not polarized.

The field of operations was now transferred to England, where, under the auspices of the British Association, four ascents were made by John Welsh of the Kew Observatory in the great Nassau balloon managed by Green, the veteran aeronaut. The special object of these investigations, like those in France, was the determination of the temperature and hygrometric condition of the air at different elevations. Besides this, samples of air at different heights were collected for analysis and the light reflected from clouds was examined for polarization. Recognizing that on account of the calm prevailing in the car of the balloon and the great solar radiation, the readings of the thermometer would be affected, Welsh enclosed the thermometers in polished tubes through which air was forced by bellows. This was the first aspirated thermometer, which alone gives the true temperature of the air with the conditions prevailing in a balloon. The instrument fell into oblivion until a few years ago, and to this fact is due the fictitious temperatures generally obtained by aeronauts. Welsh reached heights of from 12,500 to 23,000 feet, and his observations showed that the temperature of the air decreased uniformly with height until at a certain elevation, varying on different days, the decrease is arrested, and for a space of 2000 or 3000 feet the temperature remains nearly constant, or even increases slightly; the regular diminution being afterwards resumed and generally maintained at a less rapid rate than in the lower air, and commencing from a higher temperature than would have existed but for the interruption. The variation of the decrease with the seasons was also demonstrated. The humidity did not change much with height, and it was nowhere very dry. Finally, the light of the clouds was proved not to be polarized, and the permanent composition of the atmosphere was confirmed.