Page:Encyclopædia Britannica, Ninth Edition, v. 7.djvu/178

160 160 DIALLING not be used more than 4 or 5 miles north or south of the place for which they were constructed. We shall briefly describe two portable dials which were in actual use. Dial on a Cylinder. A hollow cylinder of metal (fig. 7), 4 or 5 inches high, and about an inch in diameter, has a lid which admits of toler ably easy rotation. A hole in the lid receives the style, shaped some what like a bayonet ; and the straight part of the style, which, on account of the two bends, is lower than the lid, projects hori zontally out from the cylinder to a distance of 1 or 1^ inches. When not in use the style would be taken out and placed inside the cylinder. A horizontal circle is traced on the cylin der opposite the pro jecting style, and this circle is divided into 36 approximately equi distant intervals. 1 These intervals represent spaces of time, and to each division is assigned a date, so that each month has three dates marked as follows : January 10, 20, 31 ; February 10, 20, 28 ; March 10, 20, 31 ; April 10, 20, 30, and so on, always the 10th, the 20th, and the last day of each month. Through each point of division a vertical line parallel to the axis of the cylinder is drawn from top to bottom. Now it will be readily understood that if, upon one of these days, the lid be turned so as to bring the style exactly opposite the date, and if the dial be then placed on a horizontal table so as to receive sun-light, and turned round bodily until the shadow of the style falls exactly on the vertical line below it, the shadow will terminate at some definite point of this line, the position of which point will depend on the length of the style that is, the distance of its end from the surface of the cylinder and on the altitude of the sun at that instant. Suppose that the observations are continued all day, the cylinder being very gradually turned so that the style may always face the sun, and suppose that marks are made on the vertical line to show the extremity of the shadow at each exact hour from sunrise to sunset these times being taken from a good fixed sun dial, then it is obvious that the next year, on the same date, the sun s declination being about the same, and the observer in about the same latitude, the marks made the previous year will serve to tell the time all that day. What we have said above was merely to make the principle of the instrument clear, for it is evident that this mode of marking, which would require a whole year s sunshine and hourly observation, cannot be the method employed. The positions of the marks are, in fact, obtained by calculation. Corresponding to a given date, the declination 1 Strict equality is not necessary, as the observations made are on the vertical line through each division-point, without reference to the others. It is not even requisite that the divisions should go completely and exactly round the cylinder, although they were always so drawn, and both the.se conditions were insisted upon in the directions for the consrtraction. of the sun is taken from the almanac, and this, together with the latitude of the place and the length of the style, will constitute the necessary data for computing the length of the shadow, that is, the distance of the mark below the style for each successive hour. We have assumed above that the declination of the sun is the same at the same date in different years. This is not quite correct, but, if the dates be taken for the second year after leap year, the results will be sufficiently approxi mate. The actual calculations will offer no difficulty. When all the hour marks have been placed opposite to their respective dates, then a continuous curve, joining the corresponding hour-points, will serve to find the time for a day intermediate to those set down, the lid being turned till the style occupy a proper position between the two divisions. The horizontally of the surface on which the instrument rests is a very necessary condition, especially in summer, when, the shadow of the style being long, the extreme end will shift rapidly for a small deviation from the vertical, and render the reading uncertain. The dial can also be used by holding it up by a small ring in the top of the lid, and probably the verticality is better ensured in that way. Portable Dial on a Card. This neat and very ingenious dial is attributed by Ozariam to a Jesuit Father, De Saint ttigaud, and probably dates from the early part of the 17th century. Ozanam says that it was sometimes called the capuchin, from some fancied resemblance to a cowl thrown back. Construction. Draw a straight line ACB parallel to the top of the card (fig. 8) and another DOE at right angles to it ; with C as Fig, , centre, and any convenientradius CA, describe the semi-circle AEB below the horizontal. Divide the whole arc AEB into 12 equal parts at the points r, s, t, &c., and through these points draw perpendi culars to the diameter ACB, these lines will be the hour lines, viz., the line through r will be the xi ..i line ; the line through s the x...ii line, and so on ; the hour line of noou will be the point A