DYNAM

E, one In the direction ea, and the other In the directio Ed. The first of these will appear to move with tli sun in the direction A B, and the second in the opposiif direction B A; they will approach each other until S a becomes exactly parallel to A C, when they will coincide; after this, as the angle S a A becomes greater thai i 6(P, they will separate, and gradually pa off the field, one at B, and the other at A. Hence it is evident that if A C be in the plane of the meridian, the instant of the coincidence of the two images will be that of apparent noon. When used as a meridian instrument, the dipleidoscope must be fixed so that one of the reflecting plam A C, is exactly vertical, and also in the meridian. Th latter adjustment may he made by any of the means whi( suffice for fixing a sun-dial, or for drawing a meridiai; line; the former, by suspending a fine plumb-line a fev feet before the transparent gla , and moving the instrument slowly about the edge A C till the two images of the line become exactly parallel. These are the only cs sential adjustments; but it is convenient also to fix tin instrument so that the lines of intersection of the thi <■' planes may be parallel to the axis of the earth's rotati!: in order that the sun's greatest altitude above the plain the instrument at midsummer may be equal to his great depre ion below it at midwinter. On account of ti intense light of the sun, the eye must be protected b\ coloured gla ; and the field of view may be magniti by a small telescope. Three observations should made; the first, of the instant at which the two imaL come into contact; the second, of the instant at whi they exactly coincide; and the third, of the instant ai which they separate. The instrument, if accurately made and fixed, is capable of giving true time with iii a fraction of a second. The dipleidoscope is a patent instrument, the property of Mr. Dent, chronometer maker, of the Stran<l, who has published a popular description of it in a small pamphlet, in which he states that the idea was first suggested by Mr. Bloxam, but that the original form has been much improved and simplified. From its small size, and the facility of the observation, it may be con veniently employed for the regulation of time-keepers in circumstances under which recourse cannot be had tu a transit instrument. [s. 1379]