An introduction to astronomy: designed as a text-book for the use of students in college. By Denison Olmsted ...

TI DlES. 165
at the time of observation must be k1nown, in order to estimate
tlhe effects of parallax and refraction.
2178. Since the invention of tile magnetic telegraph, it has
been employed to determine the difi'erences of longitude 1)ctween fixed stations on land \'ith a prcisiont \which was before
altogether unatttainable. Suppose two stations to be conllcted
by the tclegraphic line, and that there is fat each a clock keeping
the} local tilme. The observers agree tbeforllhand at whlat time
by his ownv clock the one at the most easterly station shall conmetlce l gi viing sicgnals; and also at twhat time the other,shall
conllnence giving anotherl series accordincg to /tis clock. I'll
interval between successive signals is also previously determilned. Wtllen the moment arrives,C the first observer strikes
tlhe tclegriaphic k<ey at thle xact beat of the clock, and thle
second observer records tlhe thne, of the signal as shown l)by hlis
own clock, and thus thlecy contimue to do till the fall series is
recorded. The, second observer then commences sendling siglnals, wllich are in like mamler recorded by the first.. The vrclocity of the electric current is so great thtat the absolute time
of makingl o a signal at one statioll, alnd of perceiving it at tlhe
otherl, may be colsideced identical, so tllat the difference which
is indicated by tlhe two clocks in cach case is wholly due to dif..
ference of longitude. Still greater lprcision is attaind by
causing the signal key at eachl station to record its own movement on the lile of second-marks made by the clock at the
other station (Art. i25i).
TI )DES.
27. I.I The tides are an alternate rising and falling of tlhe
iwatersl of thle ocean at regular intervals. I1They lhave a maximm ll and lninimm twice a day; and theJ daily rnaximltl and
minimnum eachll their lighest and lowe:c, values twice in1 a sy1nodical revolutioll of the moo1n, or 293 days. l.hle maximum
of the dlailfy tide is called Aig/ tite, and the minimum low tide.
lThe maximum tide during a lunation is callcd thle vpriig tiide,,
tl mintlilnlllll fap tile. The risingl of the tide is calledlJootl,
Silandl thim filling, we b.
Sitmiilal tides, wivettlhr high or low\, occur onl opposite sides