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32 METEOROLOGY Peters, Temple, Leverrier, and other astrono- mers calculated the path on this assumption, and then they inquired whether any known comet possesses a similar path. By a singular coincidence, a telescopic comet had been found that very year, 1866, which traversed an orbit so near to that obtained for the meteors as to leave no doubt of the identity of the two or- bits. The comparison is as follows : ELEMENTS. November meteors. Temple't comet. Perihelion distance 0-9893 0-9765 Eccentricity 0-9033 0-9054 8emi-axis major 10-840 10-324 Inclination . . ... 18 8' 7 18-1' Longitude of descending node . . Period 51 28' 88-25y. 51 26-1' 83-176y. Motion. Retrograde. Retrograde. But the matter was removed from the region of mere probability by the researches of Prof. Adams, the well known English astronomer. Analyzing the perturbative effects of the plan- ets upon the members of the November me- teor system, on the various assumptions point- ed out by Prof. Newton as mentioned above, he found that the actual changes taking place in the position of the meteors' node (changes indicated by the gradual alteration of the date of the shower) imply an orbit extending so as to bring the meteors under the disturbing influence of the giant planets. Hence the re- currence of great displays thrice in a century can only be explained by the last assumption of Newton, assigning to the meteors a period of 33^ years or thereabouts. Adams selected a period of 33 years, and found the nodal changes satisfactorily accounted for. Since then the identity of another system, the mete- ors of Nov. 27-29, so far as their path is con- cerned, with the short-period comet called Bie- la's, has been satisfactorily demonstrated, by the occurrence of a shower (predicted on that assumption) on Nov. 27, 1872. More than 100 meteor systems are now recognized, not in all or in most cases by the periodic recurrence of great displays, but by the existence of distinct radiant points. Even 10 or 12 meteors only, seen on the same night, can be safely assigned to a single system, when they are all found to radiate from nearly the same point of the star sphere. There is every reason to believe that meteoric astronomy is as yet only in its in- fancy, and that the combined study of meteor systems and comets will throw great light on many most interesting subjects of astronomi- cal research. Some of the researches of Prof. Kirkwood into the relations presented by com- ets seem very promising in this respect. METEOROLOGY (Gr. pertupof, lofty, and U- yof, discourse), the description and explana- tion of the phenomena peculiar to the atmos- phere of the earth. On the atmosphere and its changes depend the development of life, both vegetable and animal, the currents and the navigation of the ocean, and even the great changes that have been wrought in geo- logical ages by superficial disintegration and erosion. The consideration of these and kin- dred subjects gives rise to branches of science that may be considered as applications of me- teorology proper, which should be restricted to the simple consideration of the atmospheric phenomena themselves, and the laws which pro- duce them. As a science of observation, gener- alization, and induction, our present knowledge of meteorology dates from Aristotle; but as a deductive science, and one deserving to be ranked with astronomy, chemistry, and phys- ics, its history is confined to the past 25 years. In this article we shall present in brief some of the more important general statistics, thus representing the results of the observations by which deductive theories must be tested, and shall conclude with a few words on the latter. INDUCTIVE METEOROLOGY. Our review of the inductive science will be divided into sec- tions on the constitution, the temperature, the movement, the moisture, and the pressure of the atmosphere, in which arrangement we fol- low the very valuable treatise of Schmid. 1. Constitution and Properties of Air. To Priest- ley and Scheele (1774) we are indebted for our first knowledge of the chemical constitution of the atmosphere as a mixture of oxygen and nitrogen; to these constituents Bergman, in the same year added carbonic acid gas as the third component ; of the other gases that are present in very small quantities, excepting the vapor of water, it is not necessary to make further mention. The proportions by weight of the previously mentioned gases in the air over the Atlantic ocean are very nearly as follows : nitrogen, 77 per cent. ; oxygen, 23 per cent. ; carbonic acid gas, V of 1 per cent. The weight of one litre (61*027 cubic inches) of air, at 32 F., and a barometric pressure of 29-9 inches, is 1-293187 grammes (19-9569 grains), as determined by Eegnault (1847). A unit's volume of dry air at the temperature of 32, if free to expand, increases to 1-3665 on being heated to the temperature of 212. The increased pressure experienced on heating from 32 to 212 a volume of dry air confined within the same space is in the ratio of 1 to 1*36706. The specific heat of air is usually assumed as unity, and the increase of temperature due to a sudden condensation of any portion of it into a smaller volume is about 10 for a diminution in volume of ^, assuming that the air originally is at a temperature of 32 and a pressure of 30 inches. The coefficient of viscosity of air is, according to Maxwell (1872), in British mea- sures, 0-0825 at temperature 32. The altitude and figure of the atmosphere are terms to which, strictly speaking, no exact definition can be given, since it is not yet certain that the rare- fied gases in its upper portion do not merge by insensible degrees in the ether of interstellar space. On the other hand, the only portions of the atmosphere that have any important bearing upon meteorological phenomena are