2Z3

elpecially if the transits of the sun’s center is obferved by applying aruler with sights, either plain or telescopical, to the wall or plane,whose declination is required.—In drawing a meridian line, and Und-ing the magnetic Variation. — In finding the hea rings of places in ter-re llrial surveys; the transits of the sim over any place, or his horizontaldistance from it being obferved, together with the altitude and hour.—And thence determining small differences of Iongitude.—In observingthe Variation at sea, &c.

The learned Mr. Andrew Reid invented an instrument several yearsago, for finding the latitude at sea from two altitudes of the sim, ob-served on the fame day, and the interval of the observations, meafuredby a common watch. And this instrument, whose only fault was thatof its being somewhat expensive, was made by Mr. Jackson. Tablesha ve been lately computed for that purpose.

But we may osten, from the foregoing rules, resolve the fame pro-biern without mueh trouble; efpecially if we suppose the master ofthe sliip to know within 2 or 3 degrees what his latitude is. Thus,

Assume the two nearest probable limits of the latitude, and by the

theorem H — compute the hours of observation for both sup-

positions. If one interval of those computed hours coincides with theinterval obferved, the question is solved. Is not, the two distances ofthe intervals computed, from the true interval, will give a proportionalpart to be added to, or subtracted from, one of the latitudes assumed.And if more exactness is required, the Operation may be repeated withthe latitude already found.

But whichever way the question is solved, a proper allowance is tobe made for the difference of latitude arising from the ship’s courfe inthe time between the two observations.

Of the double horizontal dlal ; and the Babylonian and Italian dlals.

To the gnomonic projection, there is sometimes added a ßereographicprojection of the hour-circles, and the parallels of the sun’s declination,on the fame horizontal plane the upright side of the gnomon beingiloped into an edge, standing perpendicularly over the center of theprojection: so that the dial, being in its due position, the Ihadow ©fthat perpendicular edge is a vertical circle pasiing thron gh the sun, inthe stereographic projection.

The months being duly marked on this dial, the sun’s declination,and the length of the day at any time, are had by inspection (as alsohis altitude, by means of a scale of tangents.) But its chief property

H h is,