MEKIDIAN CIRCLE. 342 MERIMEE. fixed opposite to the rim, the portion of the in- terval to the nearest division on the rim can be read to seconds. There are sometimes six such microscopes fixed opposite diirercnt points of the rim; and the reading of the instrument is the mean of the readings of all the microscopes. This tends to eliminate errors arising from imperfect graduation and errors of observation. If the inslrunieiit is ))roperly adjusted, the zero point of the circle will be opposite the fi.ed pointer when the line of collimation of the telescope points to the zenith. In practice, however, this is not always accurately, or even approximately, the case, and is really of no consequence, as the final result of every observation is the difference between two readings. It is evident that the ditrerence between any two readings of the instru- ment will represent the angle through which the line of collimation of the telescope moves in pass- ing from one position to the other. It remains to show how a fixed point, viz. the nadir (q.v.), is observed, and then how an observation is taken of the star itself on its meridian passage. It must be explained here that the fixed horizontal wire in the eye-piece of the telescope, in the instru- ment as now used, is only an imaginary line, which determines the line of collimation of the telescope. It coincides with the position of the micrometer wire when the screw-head of the micrometer marks zero. To observe the nadir, a trough of mercury is placed underneath the instrument, and the tele- scope is turned so as to look vertically down- ward into it. An image of the system of cross- wires which is in the common focus of the object- glass and eyepiece will be rellected back again to nearly the same focus. Looking into the tele- scope, the observer now- adjusts it by means of a slow-motion screw till the rellected image of the horizontal wire coincides with the real one. The final adjusttnent is perhaps most delicately ef- fected by turning the screw-head of the microme- ter which moves the wire itself. When they coincide, the line joining the centre of the object- glass of the telescope with the intersection be- tween the middle vertical and horizontal micro- metric wire will be vertical. For that position of the movable wire, the circle now gives the exact nadir reading, which difTers 180° from the true zenith reading. Again, to ob.serve a star in the meridian, the instrument is previously adjusted so that the star, in passing the meridian, shall pass over the field of view of the telescope. As the image of the star approaches the centre of the field, the ob- server adjusts the telescope by the slow-motion screw, so as very nearly to bring the image of the ■ star to the horizontal wire. Finally, just as the star passes the middle vertical w^ire, he bisects the image of the star with the hori- zontal wire l)v a touch of the micrometer screw- head. The circle being now clamped (or made fast), the reading is determined as before by reading the pointer and microsco])es. and adding or subtracting, as the case may be, the reading of the micrometer. This reading now subtracted from the zenith reading gives the meridian zenith distance of the star; and this, again, subtracted from !in°, gives its meridian altitude above the horizon. See Tiun.sit Tnstri'MENT. MEKIDIAN MEASUREMENT. The deter- iiiitKilinn I'f till' form and si/c nf the earth from the measurement of a meridional arc' has lucn a favorite problem with mathematicians from the earliest times, butf up to the middle of the eigh- teenth century their operations were not carried on with exactness sufficient to render their con- clusions of much value. Since that time, how- ever, geodesy has progressed so rapidly, owing to the invention of more accurate instruments and the discovery of new methods, that the measure- ment of the meridian can now be ])erformed with very high precision. The modus operandi is as follows: Two stations, having nearly the same longitude, are chosen ; their latitude and longi- tude are accurately determined (the error of a single second in latitude introduces a consider- able error into the result ) , and the direction of the meridian to be measured ascertained; then a base lino is measured with the greatest accuracy, as an error here generally bccumes increased at every sub-sequent ste]) ; and then, by the method known as triangulation (q.v. I, the length of the arc of the meridian contained between the paral- lels of latitude of the two stations is ascertained. , As the previously found latitudes of its two ex- i tremities give the number of degrees it contains, ; the average length of a degree of this arc can , be at once determined; and also, on certain as- sximptions as to the earth's form, the length of ' the whole meridional circumference of the earth. This operation of meridian measurement has l)ecn performed at dillerent times on a great many arcs lying between latitude 08° N. and latitude 38° S., and the results show a steady though irregular increase in the length of the degree of latitude as the latitude increases, thi the sup- position that the law of increase holds good to the poles, the length of every tenth degree of latitude up to 70° is as follows: DEGBEE OF LATITUDE Length o( depree In English J<*t 0° 3f.2.-5G 10° 362,868 20° 363.188 30° 363,679 40° 3114,284 60° 3M.929 60° ' 365. 53G 70° 3(16.033 This table is calculated on the theory that the earth is not s]dierical, as in that case the length of all degrees of latitude would be alike, but of a more or less spheroidal form, that is, having its curvature becoming less and less as we go from the extremity of its greater or equa- torial diameter to the lesser or polar axis. See Earth : Uegree of Latitide. .Mekidio.nal 1'akts. In preparing a chart (q.v.) upon Merca tor's projection, in order to preserve the relative proportion between the lengths of meridians aiirl parallels at any point, the former must be increased in jengtli. The lengths of small portions of the meridians thus increased are called miridinmil /)((^^«. and tables giving the lengths at dilTerent latitudes are pre- lianil for use in constructing charts. MERIMEE, mft'r.'-niA'. Prosi-kk (1803-70). A French nnvclist, historian, diamalist, and critic, born in Paris, September 28, ISOH. He studied law, which he never practiced; he held various olTices in the civil service, bccnine. in 1831, inspector of archieological and historical monuments of France, an .Academician in 1844, and a Senator of the Empire in 18.53. His re-
