DAWSON, JOHN WILLIASI, LL.D., b. 1820, in Picton, Nova Scotia; a geologist; graduate of the university of Edinburgh. In 1841, be made explorations in Nova Scotia, and describes its geology in Proceedings of the Geological Society of London. In 1850. he was superintendent of education in Nova Scotia, and in 1835, became principal of McGill college at Montreal. Some of his works arc Handbook of the Geography and _Natural History of _Nora Scotia; _Hints to the Farmers of _Nova Scotia; Acadian, Geology; Arehaia, or Studies of the Cosmogony and _Natural History of the Hebrew Scriptures; the Story of the Earth and Nan, a treatise on geology. Ile has also written many geological articles in the Proceedings of the Geological Society of London, The Canadian _Naturalist, and other journals.
DAX, a t. of France, in the department of Landes, pleasantly situated on the left bank of the Adour, m. n.e. of Bayonne. Among its principal buildings arc the high church, once a cathedral, and the bishop's palace. It is an intermediate depot for goods forwarded to Spain, and has some manufactures of earthenware, wine, and brandy; but it is chiefly remarkable for its hot saline springs, the temperature of which at the source is 158' F. The water, which is medicinal and nearly tasteless, was used for bathing purposes by the Romans, who conferred upon the springs the name Aqure Augusta Tarbelliew. Pop. '76, 9,085.
DAY (probably allied to the Lat. dies, day , divum, the sky, from the root dlv, to shine) originally meant the space of time during which it is light, in opposition to the space of darkness or night; it now more usually denotes a complete alternation of light and dark ness. It is the earth's rotation that causes the vicissitude of day and night. The earth being a globe, only one half of it can be in the sun's light at once; to that half it is day, while the other half is in its own shadow, or in night. But by the earth's rotation, the several portions of the surface have each their turn of light and of darkness. This hap pens because the position of the earth is such that the equator is on the whole presented towards the sun; had either pole been towards the sun, that hemisphere would have revolved in continual light, the other in continual darkness.
One complete rotation of the earth does not make a day, in the usual sense. If the time is noted when a particular fixed star is exactly s. or on the meridian, when the same star comes again to the meridian the next day, the earth has made exactly one rotation, and the time that has elapsed is called a sidereal day. This portion of time is always of the'same length; for the motion of the earth on its axis is strictly uniform, and is, in fact, the only strictly uniform motion that nature presents us with. Sidereal time, or star-time, from its unvarying Uniformity, is much used by astronomers. But the pas
sage of a star across the meridian is not a conspicuous enough event for regulating the movements of men in general. It is not a complete rotation of the earth. but a com plete alternation of light and darkness that constitutes their day. This, which is called the civil or the solar day, is measured between two meridian passages of the sun, and is about four minutes longer than the sidereal day. The cause of the greater length is this: When the earth has made one complete turn. so as to bring the meridian of the place to the same position among the fixed stars as when it was noon the day before, the sun has in the meantime (apparently) moved eastward nearly one degree among the stars, and it takes the earth about four minutes more move round so as to overtake him. If this east -ward motion of the sun were uniform, the length of the solar Any would-be as simple and as easily determined as that of the sidereal. But the ecliptic or sun's path crosses the earth's equator, and is therefore more oblique to the direction of the earth's rotation at one time than another; and besides, as the earth moves in her orbit with varying speed, the rate of the sun's apparent motion in the ecliptic, which is caused by that of the earth, must also vary. The consequence is, that the length of the solar day is constantly fluctuating; and to get a fixed measure of solar time, astronomers have to imagine a still moving uniformly in the celestial equator, and completing its circuit in the same time as the real sun. The time marked by this imaginary sun is called mean solar time; when the imaginary sun is on the meridian, it is nzean noon; when the real sun is on the meridian, it is apparent noon. It is obvious that a sun-dial must show apparent time, while clocks and watches keep mean time. Only in four days of the year do these two kinds of time coincide. In the intervals, the sun is always either too fast or too slow; and the difference is called the equation of time, because, when added to or substracted from apparent time, it makes it equal to mean time. The mean solar day is divided into twenty-four hours, the hours into minutes and seconds. A sidereal day, we have seen, is shorter; its exact length is 23 hours, 56 minutes. 4 seconds of mean solar or common time Astronomers divide the sidereal day also into twenty-four hours, which arc of course shorter than common hours. In the course of a civil year of 365 days, the earth turns on its axis 366 times, or there are 366 sidereal days. Astronomers reckon the day as beginning at noon, and count the hours from 1 to 24. The civil day begins at midnight, and the hours are counted in two divisions of twelve each. The ecclesiastical day was reckoned from sunset to sunset.