222 AIR PUMP AIRY low, opens to let the latter pass, closing at the same time the pressure and suction pipes ; then the lower cock w will become the upper, and while closing perform its func- tions. The wheel turns on two hollow trun- nions, the one in front being connected with the suction pipe, the one behind with the press- ure pipe. During rotation the axis will thus perform a continuous suction and pressure, which in order to be considerable requires a wheel of large dimensions ; 30 inches differ- ence in the height of the mercury at the two sides corresponding with our atmosphere, a wheel of at least 5 feet diameter is required to produce a vacuum, while if pressure is also re- quired, double and triple these dimensions must be given. The inventors had recently such ap- paratus in operation with a wheel of 16 feet diameter and containing 2,000 Ibs. of mercury. Air pumps are used by professors of natural philosophy, to show that in a vacuum combus- tion is arrested, smoke falls like lead, cold wa- ter boils, warm-blooded animals die rapidly, fermentation is stopped, &c. The celebrated process of Appert for the preservation of ali- mentary substances is founded on the last mentioned property ; but the necessary vacuum is produced, not by using an air pump, but by boiling the boxes of preserves, thus producing steam that expels the air, and then quickly sol- dering up the hole while the steam still fills up the space, and before the air is given time to enter ; the vacuum will be produced after cool- ing while the steam is condensed to water. Air pumps are at the present day also used in many manufactories. The sugar refiners use it for the rapid evaporation of the syrup at low temperatures ; and the condensation of milk is performed by means of large air pumps. The artificial manufacture of ice, and artificial cool- ing by the use of power, are always accom- plished by the intervention of powerful air pumps, whether air itself is alternately expand- ed and compressed, or use is made of volatile liquids, as ether, ammonia, and chymogene, which by evaporating in a vacuum produced by the air pump generate the most intense arti- ficial cold. Many chemical preparations also require the constant use of a vacuum, or at least of very rarefied air, for which reason the air pump is one of the most important tools in all manufactories of chemicals, as well as in the chemical laboratory. Recently the use of the air pump has been introduced for the pres- ervation of wood and other porcos material, by first exhausting the air from the pores, so as to force the preserving liquids in by atmos- pheric pressure. For manufacturers of aneroid barometers, sympiesometers, Geisler's tubes, and other physical instruments, the air pump is also an indispensable tool. Finally, one of the most important applications of th0 air pump is that to the low-pressure steam engine ; it is used to pump out of the condenser the condensed steam, the water introduced for condensing, and the air that has come out of this water when warmed by the condensation of steam. This application of the air pump is one of the inventions of Watt. AIR VESSELS, or properly Spiral Vessels, are supposed by some botanists to be the only for- mation by which air is conveyed into the ve- getable system; but air has access to many parts of the plant by means independent of the spiral vessels. Spiral vessels differ from spiral cells (or vermiform bodies) only by dimension, so that there is a constant transition from the latter into the former. Both are quite as fre- quently filled with sap (in the youngest por- tions of the plant) as with air (in the full-sized organs). They are first perceptible in the bud. The spiral vessels of the wood are to be dis- tinguished from those of herbaceous plants, both as regards their origin and their function. The latter has not yet been fully explained, owing to the diversity of views entertained by different inquirers. Spiral formation begins when the simple cell membrane ceases to ex- ist. This, as well as all other transitions from one form to another, is accompanied by modi- fications and changes of the chemical constitu- ents of the vegetable body. In some cases the air in the cavities of the plant contains more oxygen than the atmospheric. AIRY, George Blddell, astronomer royal of Eng- land, born at Alnwick, July 27, 1801. He was a fellow of St. John's college, Cambridge, and afterward of Trinity. He was appointed Plu- merian professor of astronomy at Cambridge in 1828, and annually published the results of his investigations at the observatory there (9 vols. 4to, 1829-'38). On the retirement of Mr. John Pond in 1835, he became astronomer royal, which office he continues to hold (1872). He is president of the astronomical society, and in 1871 was elected president of the royal society. He distinguished himself by the pub- lication of the long-neglected observations at Greenwich of the moon and planets from 1750 to 1830. His labors are regarded as having opened a new epoch in planetary astronomy. He has acquired high reputation by his re- searches into the mode of simplifying the the- ory of planetary perturbation, by his con- trivance of a new instrument for observing the moon off the meridian, and substituting for the old mural circle and transit instrument another of simple construction and of great utility. In 1854 he corrected certain erroneous impres- sions which prevailed touching the variations of the compass in ironclads. In 1859 his mem- orable researches on the motion of the solar system in space were first announced. He prepared the formula and methods for con- ducting the survey of the Maine boundary be- tween Canada and the United States. He observed eclipses of the sun at Turin (1842), at Gothenburg (1857), and in Spain (I860). His important contributions to astronomy, magnetism, meteorology, photography, and other sciences are contained in leading English cyclopaedias and in the annals of learned socie-
