If, then, the apparatus be so suspended that the circle, M, shall be in the meridian, the slide parallel with the earth’s axis, and the circle, E, at right angles with the slide, the pencil of solar light passing through the aperture will describe, in one day, a cone having the slide for an axis; that is to say, concentric with the equator circle.  If, moreover, the aperture is properly placed, the luminous pencil will pass through the equator circle itself; to this effect, the aperture should be in a position such that the angle, a (Fig. 3, No. 4), may be equal to the declination of the sun on the day of observation.  It is precisely to this end that the names of the months are inscribed upon the slide....

[Illustration:  FIG. 1.—­TRAVELER’S SUN DIAL.]

The accessories of the instrument are as follows:  A ring with a pivot for suspending the meridian circle, and the position of which, given by a division in degrees marked upon this circle, must correspond with the latitude of the place; two stops serving to fix the position of the equator circle; finally the latitude of various cities.  The instrument was constructed at Paris, by Butterfield, probably in the last quarter of the eighteenth century.

The second instrument, which is of the same nature as the cubical sun dial—­that is to say, with horary angle—­is, unlike the latter, a true trinket, as interesting as a work of art as it is as an astronomical instrument.  It is a little mandolin of gilded brass, and is shown of actual size in Fig. 2.  The cover, which is held by a hook, may be placed in a vertical position, in which it is held by a second hook.  It bears in the interior the date 1612.  This is the only explicit historic datum that this little masterpiece reveals to us.  Its maker, who was certainly an artist, and, as we shall see, also a man of science, had the modesty not to inscribe his name in it.

[Illustration:  FIG. 2.—­SUN DIAL IN THE FORM OF A MANDOLIN, CONSTRUCTED IN 1612.]

No. 2 of Fig. 3 represents the instrument open.  It rests upon the tail piece and neck of the mandolin.  The cover is exactly vertical.  The bottom of the mandolin is closed by a horizontal silver plate, beneath which is soldered the box of a compass designed to put the instrument in the meridian, and carrying upon its face an arrow and the indications S. OR.  M. OC., that is to say, “Septentrion” (north), “Orient” (east), “Midi” (south), “Occident” (west).  One of the ends of the needle of the compass is straight, while the other is forked.  It is placed in a position in which it completes the arrow, thus permitting of making a very accurate observation (Fig. 2, No. 3).  Around the compass, the silver plate carries the lines of hours.  It is perfectly adjusted, and held in place by a screw that traverses the bottom of the instrument.  In front of the compass it contains a small aperture designed to permit of the passage of the indicating thread, which, at the other end,