YAWNING- -YELL 



139 



YAWNING, OR GAPING; an involuntary 

 opening of the mouth, generally produced by weari- 

 ness or an inclination to sleep, sometimes by hun- 

 ger, sympathy, &c. It often precedes the fit in 

 some intermittent fevers, and, in some instances, 

 by the frequency of its recurrence, becomes a real 

 disease. It is supposed to be determined by an 

 interruption of the pulmonary circulation. Yawn- 

 ing, according to Boerhaave, is performed by ex- 

 panding at one and the same time all the muscles 

 capable of spontaneous motion, by extending the 

 lungs, by drawing in, gradually and slowly, a large 

 quantity of air, and gradually and slowly expiring 

 it after it has been retained for some time, and then 

 restoring the muscles to their natural state. 

 Hence the effect of yawning is to move, accelerate, 

 and equally distribute all the humours through all 

 the vessels of the body, and, consequently, to 

 qualify the muscles and organs of sensation for 

 their various functions. When yawning is trouble- 

 some, long, deep respiration, or drawing in the air 

 at long intervals, relieves it. 



YAZOO ; a river of Mississippi, which rises in 

 lat. 35 N., near the borders of Tennessee, and runs 

 south-south-west into the Mississippi, which it 

 meets twelve miles above Walnut hills, 142 miles 

 above Natchez. It is 230 miles long, and navigable 

 100 miles. 



YAZOO LANDS. See Georgia. 



YEAR ; the period in which the revolution of 

 the earth round the sun, and the accompanying 

 changes in the order of nature, are completed. In 

 ancient times, when it was thought that the sun 

 moved round the earth, this period was called the 

 solar year. The accurate determination of the 

 solar year, which required great knowledge of as- 

 tronomy and exact observations, could only be 

 reached by the successive efforts of many genera- 

 tions. According to Herodotus, the Egyptians 

 were the first who approximated to the true length 

 of the solar year. They divided it into twelve 

 months, and each month into thirty days, so that 

 their year consisted of 360 days ; and the inhabi- 

 tants of Thebes, who did not take into considera- 

 tion the course of the moon, added five days. They 

 afterwards remarked that the dog-star (Sirius), 

 whose appearance just before sunrise denoted the 

 overflowing of the Nile, became visible one day 

 later every four years ; but the year of 365 days 

 was so intimately connected with their festivals, 

 that a change could not be made without the great- 

 est difficulty ; and, although the festivals occurred 

 later and later, yet the mode of reckoning remained 

 the same until the Romans became masters of 

 Egypt, when the calendar of Julius Caesar was in- 

 troduced. In Greece, the year was more correctly 

 divided into 365 days; and the Grecian astrono- 

 mer Sosigenes made this the basis of the Julian 

 calendar. (See Calendar.) But the astronomer 

 Hipparchus of Alexandria, about 150 years before 

 Christ, found, by observation, that the solar year 

 contained only 365rf. 5/t. 55'. His improvements, 

 however, were not adopted. Later observations 

 have shown that the true year is about 11' 15'' 

 shorter than the Julian year. Lalande made it 

 365d. 5A. 48' 35'' 30"'; Zach, 365d. 5h. 48' 48-016''. 

 This period, so accurately determined, is called the 

 astronomical year, from which the civil year of the 

 calendar must necessarily differ. As the civil year 

 cannot divide the days, it only reckons 365 in the 

 year, and therefore does not fully agree with the 

 astronomical. On account of the remaining 5A. 48", 



&c., every four years a day is added to the month 

 of February; and the year which thus consists of 

 366 days is called leap year. By the lunar year is 

 meant the time required for twelve revolutions of 

 the moon, which is, according to Lalande, 354d. 8h. 

 48' 37", making the lunar year Wd. 21A. shorter 

 than the solar. Many nations of antiquity reckon- 

 ed by the lunar year. A year is said to be fixed, if 

 the equinoxes and seasons come on fixed days; but 

 if they advance, the year is called changeable. 

 Thus the Julian year is changeable ; the Gregorian 

 fixed. It is necessary to observe the difference be- 

 tween the tropical, sidereal and anomalistic year. 

 The astronomical year is also called tropical, be- 

 cause its duration depends on the return of ^he sun 

 to the equinoxes or the tropics. This differs from 

 the sidereal year (the time required by the sun to 

 complete a revolution with regard to a particular 

 star), which is longer by 20' 5.7''; and the anoma- 

 listic year is 26' longer than the tropical, and is the 

 time required by the sun to complete a revolution 

 with regard to its apogee. The year of the Jews 

 consisted of twelve months, which were divided 

 alternately into twenty-nine and thirty days. A 

 whole month was inserted in their leap year, be- 

 tween the sixth and seventh month. Their new- 

 year's day was the day of the first new moon after 

 the autumnal equinox. In the period of nineteen 

 years, by which they reckoned, they had seven leap 

 years, namely, the third, sixth, eighth, eleventh, 

 fourteenth, seventeenth, and nineteenth. Among 

 the Persians, the sultan Gelal (A. D. 1079), intro- 

 duced a year which more nearly agrees with the 

 astronomical than the Gregorian year does. Ac- 

 cording to his arrangement, a leap year occurs once 

 in four years seventimes in succession.; the eighth 

 leap year, however, does not take place till after a 

 lapse of five years. During the time of the French 

 republic, a year was invented also more exact than 

 the Gregorian. A period of 86,400 years requires 

 20,929 leap years ; therefore a day was to be in- 

 serted at the end of the year as often as the autum- 

 nal equinox would fall on the second day of the 

 new year. For further information, see Calendar. 



YEAST is the barm or froth which rises in beer 

 and other malt liquors during a state of fermenta- 

 tion. When thrown up by a quantity of malt or 

 vinous liquid, it may be preserved to be put into 

 another at a future period, on which it will exert a 

 similar fermentative action. Yeast is likewise used 

 in the making of bread, which, without such an 

 addition, would be heavy and unwholesome. 



YEDDO. See Jaddo. 



YELL ; one of the Shetland islands, the next 

 largest of the group to Mainland, from which it is 

 separated by Yell Sound, as it is from Unst by the 

 strait of Blommel, and from Fetlar by Colgrave 

 Sound. It is about twenty miles in length, and 

 from six to twelve in breadth, exhibiting a toler- 

 ably level surface, but a bold and rugged coast, in- 

 dented by many bays or voes, of which Bura, Ham- 

 ma, Refirth, and Whalfirth are the principal, and 

 form secure harbours. There are a few small 

 patches of arable land near the shore, but the interior 

 affords only a coarse pasturage for sheep and cattle, 

 and a plentiful supply of peats, in procuring which 

 large trees are frequently discovered in the mosses, 

 though scarcely a shrub is now to be seen upon the 

 surface. The island was formerly divided into 

 three parishes, North Yell, now in union with the 

 isle and parish of Fetlar, South Yell, and Mid Yell, 

 which are also united. There are two churches. 



