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Messrs. Editors—From the notice of sundials in a late issue of the Scientific American, I am led to make the following remarks :— There is no more beautiful or ingenious instrument than the sundial; when correctly made and its use properly understood, it can present the true time with an unvarying exactitude to be found only in the works of the Divine Artificer, upon which its power depends. The only difficulty lies in the variable nature of the shadow's progress through the varying nature of the sun's course, which will give a different reading to the hour circle from the mean, or average or clock time. While the dial indicates solar time, varying with the season, the clock presents equable or mean time, being the precise or exact division of the hours and minutes to their equable length, yet there is no real Ufference between the two. They both come to the same conclusion, and both precisely accomplish in a given period their due degree. Hence with the smallest possible trouble it is easy to find the very thing sought, and at any time to discover the true clock time. The following table will answer for such indication to any person using a dial: The sun's center is on meridian, and the dial shows noon on ff. m. s. Jan. 1, whentheclocktimeshows 12 4 3 Feb. 1, " " 12 13 57 Mar. 1, " " 12 12 32 April 9, " " 12 0 0 May 9, " " 11 56 55 June 9, " " 11 57 31 " 21, " " 12 0 20 July 1, " " 12 3 29 Aug. 1, " " 12 6 00 Sept. 1, " " 11 59 46 Oct. 1, " 11 49 35 Nov. 1, " " 11 43 43 Dec. 1, " " 11 49 23 By this it will be easy to see how much difference should be allowed for the equation of time, and at any period to find the clock time by the dial indication. It must be remembered, however, that a dial to be exact must be most carefully placed. Simply setting a dial north and south is not at all sufficient. Pains must be taken to secure a true meridian, and before the dial is located, that meridian should be found with great exactness, so that in setting the dial (if horizontal) the gnomon shall be perfectly adapted to the true meridian of the place where it is to stand. It would seem that an agreeable and really useful accompaniment to the dial would be a prolongation of the horary circle, sufficient to allow the scale of signs to be inscribed, and the style to track out the sun's path through the heavens, and thus unerringly indicate his place in the ecliptic. If in either side of the astronomic signs the names of the months were written, it would be a most pleasing occupation to notice month by month the progress of the sun in his vibrations backward and forward, and to children it would show clearly the motion of that planet. K. W. [The above communication on the construction of sundials is not only interesting' but valuable, and the facts contained have the freshness of positive experiment, and are consequently of interest to our readers.—Eds.
