lar metal box which has been nearly exhausted of air and then hermetically sealed. The sides are corrugated in concentric rings, so as to increase their elasticity, and one of them is fixed to the back of the brass case which contains the whole. The amount of exhaustion is such that if the sides of the box were allowed to take their natural position they would be pressed in upon each other, and to prevent this they arc kept dis- tended, to a certain extent, by a strong spring, S, fixed to the case, which acts upon the head of the pillar, B, attached to the side next the face. When the pressure of the air increases, there be- ing little or no air inside the box to resist it, the corrugated sides are forced inward, and when it diminishes again, their elas- ticity restores them to their former place, the box being extremely sensitive to the varying pressure of the ex- ternal atmosphere. Suppos- ing the two sides pressed inward, the end of the spring, E, will be drawn toward the back of the case, and carry with it the rod, EG, which is firmly fixed into it. EG, by the link GH, acts on the bent lever, HKL, which has its axis at K, so that, while the arm, KH, is pushed to the right, LK is moved downward. By this motion a watch-chain, O, attached at L, is drawn off the little drum, M, and the index-hand, PP, which is fixed to it, would move from the position repre- sented in Fig. 1 to one toward the right. When the contrary motion takes place, a hair-spring moves the drum and the hand in the opposite way. By this or similar mechanism a very small motion of the corrugated sides produces a large deviation of the index- hand.
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FIG. 1. ANEROID BAROMETER.
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FIG. 2. CROSS-SECTION OF ANEROID.
The aneroid is graduated to represent the inches or millimeters of the mercurial barometer. It may be made to agree very closely with such an instrument, but, owing to the imperfect elasticity of the box and the steel spring, it is quite apt to disagree after a few months or years, and especially when exposed to rapid variations of pressure, being in this respect quite analogous to the ordinary thermometer, whose glass bulb has also a defective elastic reaction. Although the aneroid is very convenient, very sensitive, and unaffected by variations in gravity, yet its defects prevent it from becoming a very reliable instrument, and it must be frequently compared with the standard mercurial. It is often used in ascertaining altitude, especially by engineers and surveyors, where extreme accuracy is not required. The holosteric aneroid made by Naudet and the box-aneroid made by Goldschmid bear the highest reputation.
The Bourdon aneroid, or pressure gauge, has about the same advantages and defects as the Vidi aneroid. It consists essentially of a portion of a thin hollow ring whose section is a very flat ellipse. The ring, or curved tube, is made of elastic metal, exhausted of air and then hermetically sealed, and is, therefore, a vacuum-chamber. Changes of pressure alter the curvature of this ring, whose changes of shape are shown on a magnified scale by a delicate pointer.
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FIG. 3. BAROGRAPH, OR RECORDING ANEROID.
The aneroid barometer is also easily arranged to be self-registering, and is then known as a barograph. Instruments of this kind, made by Richard & Son, of Paris, are especially applicable for use at sea, and are also widely used at land stations. For fuller details of the construction and use of the aneroid, consult Abbe's Treatise on Meteorological Instruments (Washington, 1887), and the works referred to therein, or any of the larger treatises on experimental physics. See Barometer.
AN'EURIN. A Welsh poet, who probably lived c. 603. According to the received account, he was the son of Caw ab Geraint, the chief of the Otadini; while others have identified him with Gildas, the historian, and Mr. Stephens, the translator of his poem, makes him Gildas's son. He was present at the battle of Cattraeth as bard and taken prisoner. After his release he returned to Llancarvan, and later in life lived at Galloway. He is said to have perished at the hands of Eidyn ab Einygan. His epic poem Gododin, which in its present form contains over nine hundred lines, tells of the defeat of the Britons by the Saxons at Cattraeth: but the obscurity of the language has made it impossible to gain from it a clear account of the defeat,
