280 6EOGRAPWr ANT) ÁSTIONOMy. ProblemXXXIX. " The day and hour of a lunar eclipse being known, to find by the globe all those places in which the same will be visible." By problem XIII. find as before at what place the sun is vertical at that hour ; then by problem IV. find the antipodes of that place : Rectify the glebe for the latitude of those antipodes thus they will be in the zenith, or in the pole of the horizon ; then observe as before what places are in the upper hemisphere of the globe, for in the most of those places the moon will be visibly eclipsed. The reason of rectifying the globe for the antipodes in this problem, is because the moon must be directly opposite to the sun whensoever she is eclipsed. SECT. XX.Problems relating to Geography and Astronomy, to beperformed by the Use of the plain Scale and Compasses. IT is supposed that the reader is already acquainted with some of the first and easiest principles of geometry, before he can read with understanding this or any other treatise of astrono- my or geography ; and it is presumed also that he knows what is a chord, a tangent anda sine, and how to make and to measure an angle either by a line or scale of chords, or sines or tangents, in order to practise the problems of this ' last section ; though a very slight knowledge of these things is sufficient for this pur- pose. Because several of the following problems will depend upon the altitude, or azimuth of the sun, and in order to obtain these, we sometimes use a pin or needle fixed perpendicularly, on an upright or horizontal plane ; therefore the first problem I pro- pose shall be this, (viz.) Problem I. " How to fix a needle perpendicular on a plane, or to raise aperpendicular style or pointer in order to make obser- vationsof a shadow." Note, Any thing fixed or set up to cast a shadow is called rs style. One way to perform this, is by having at hand a joiner's square, andwhile one edge of it is laid flat to the plane, the other edge of it standing up will shew when a needleor style is fixed on that plane perpendicularly, if it be applied to the side of the needle. Note, If you have a little square made of box or any hard wood, one leg being fix, and the other eight or nine inches long, one inch or 11 broad, and an inch thick, with a thread and plum. met hanging from the end of one leg, down toward theplace where the other leg is joined, as in figure xiv. and a large hole for the plummet to play in it :,it will be ofuse not only to show you, how to erect a needle truly perpendicular; but it will alsodiscoveC' whether any plane be truly' smooth, andbe horizontal or level, as well as whether any upright plane be exactly perpendicular to the
RkJQdWJsaXNoZXIy OTcyMjk=