Page:The American Cyclopædia (1879) Volume VI.djvu/76

 68 DIAL DIAL, an instrument for ascertaining the hour of the (lav by means of rays of light comingfrom the heavenly bodies. Then- are therefore so ar, lunar, and astral dials. The sun dial only will be red in this article. It is one ot the old- est of human inventions, but its origin cannot be traced. The earliest historical mention ot it i- in the Old Testament, where we are told ,,f the miracle wrought with the dial of Ahaz, king of Judah, at the instance of the prophet Isaiah, for a sign to Hezekiah, the son of Ahaz. The instrument used by the Chaldean histo- rian and astronomer Berosus, early in the 3d century B.C., is the most ancient of whose form we have any precise account. It was a hollow hemisphere* with its convexity turned toward the earth, and a button or small globule held in the spherical centre, which, by casting a shadow in the concavity, marked the suc- ceeding hours from one rim to the opposite. It gave place to dials which required more mathematical knowledge for their construc- tion ; but a hemisphere properly adjusted will answer the purpose of a dial very well, and was used by several nations long after the commencement of our era. Four have been discovered in modern times in Italy. One found at Tivoli in 1746 is supposed to have be- longed to Cicero. Another was found in 1762 at Pompeii, and as it was adapted to the lati- tude of Memphis, it has been ascribed to the Egyptians, although no sun dial has ever been discovered among the ruins of Egypt, nor have any representations of it been found among their sculptures. It is believed, how- ever, that they used some form of sun dial, notwithstanding the care they bestowed upon clepsydras, and their probable use of the pen- dulum. Perhaps the obelisks erected in honor of the sun were used as gnomons, and it has been suggested that the famous circle of Osy- mandyas might have been used to determine the azimuths of the heavenly bodies, and therefore the hours of the day. The tower of the winds at Athens, which from its architec- ture is judged to be of a somewhat later date than the time of Pericles, is an octagonal struc- ture, and bears eight sun dials for the cardinal anil intermediate points of the compass. They are described in Stuart's " Antiquities of Athen-." Four others, known as the dials of Phivdrus, also found at Athens, are now in the British museum, and are described in Delam- bre's ffutoire de Vcutronomie ancienne. Their iction shows that the Greeks used geo- il mi-thuds for vertical and also for de- clining dial-. The first sun dial said to have been erected in Homo was by L. Papirius Cur- ' ho had taken it from the Samnites. About U<> years after anoth.-r was placed near M. Valerius Mei-ala. who brought it from Sicily in the second year of the first Punic war. - made for the latitude of Catania, 4^ houth of Home. The first dial constructed at . and adapted to its latitude, is said to have been by the order of Q. Marcius Philip- pius, in 164 B. C. The sun dial may have many forms, depending upon its position in regard to the sun, and upon the latitude in which it is used. The most common form at the pres- ent day is the horizontal dial. It consists of a horizontal plate upon which the hours are marked, and which supports a style or gnomon for casting the sun's shadow, having its edge parallel with the axis of the earth. Another form is that called the equinoctial dial, con- sisting of a staff or gnomon placed parallel with the axis of the earth, and passing per- pendicularly through the centre of a circle divided into equal parts for marking the hours. The slight deviation of the sun's apparent from the true time is not taken into account in the construction of dials, the correction being made after the observation is taken. Let fig. 1 rep- FlG. 1. resent the earth, K the north pole, A B the equator, and a, ft, e, d, e, meridian lines; a, e, and e marking the quadrants. As the earth revolves on its axis with uniform motion once in 24 hours, each point moves through 15 every hour ; therefore, if it is noon on the meridian a, in three hours after it will be noon on the meridian b, 45 from a, and in six hours the meridian e, marked 90, will be brought vertically beneath the sun. All the rays of the sun which strike the earth are apparently par- allel, because of his immense distance, which is about 12,000 times the earth's diameter. It will therefore be 6 o'clock in the morning on the meridian c when it is noon on the merid- ian a. Suppose a circle to be placed at the pole, in the plane of the horizon, and divided into degrees corresponding with those on the equator, and a staff which shall represent an extension of the earth's axis to pass through its centre ; it follows that when the sun's path is