308 



HYDROGRAPHICAL SURVEYING [chap. xiii. 



Error 

 obtained 

 of Hack- 

 Watch 



Practical 

 Rule for 

 Calcula- 

 tion of 

 Equation. 



In making the observations it is most convenient to ascer- 

 tain the Error of the hack-watch, and thence, by using the 

 comparisons, to arrive at the Error of the standard. In the 

 case where the watch has a large rate, as shown by the com- 

 parisons before and after sights, the elapsed time must be 

 corrected for the amount gained or lost by the watch in the 

 intt?rval on mean time, which can be rouglily calculated from 

 the known rate of the standard. 



Having meaned the sights, and obtained the mean middle 

 time for each set, and knoA\ing the estimated latitude and 

 longitude, the rule for working a set of equal altitudes at 

 superior transit will stand thus : 



1. Ascertain elapsed time by subtracting the central time 

 of observation before transit from the central time after transit, 

 increased, if necessary, by twelve hours. Halve this, ani if 

 there are decimals, they can be rejected, the nearest even second 

 being used. 



2. To hours apply longitude to find Greenwich date of 

 apparent noon at place. 



3. Correct declination at apparent noon in " Nautical 

 Almanac " for the Greenwich date of apparent noon. 



4. Correct equation of time at apparent noon for Greenwich 

 date of apparent noon. 



5. Multiply the variation in declination for one hour by the 



c 

 half-elapsed time, to get . 



N.B. — The variation we want is that at Greenwich time of 

 local noon ; we must therefore correct the variation given in the 

 " Nautical Almanac " for the longitude. 



It is worth noting that in turning the minutes and seconds 

 of I E.T. into the decimal part of an hour, an exact and more 

 easily workable decimal may be obtained by altering | E.T. by 

 a few seconds. The equation of equal altitudes is practically un- 

 affected by so doing, if the log. cosecant and log. cotangent of 

 the amended | E.T. are used in the calculation, because the 

 o nission or addition of a single observation would of itself 

 alter | E.T. by a greater amount than that suggested. 



I E T. will thus be expressed to two places of decimals 

 instead of three, giving fewer figures by which to multiply the 



