346 PHILOSOPHICAL TRANSACTIONS. [anNO 166Q. 



the morning, when the sun is just half above the horizon, note what hour, min. 

 and sec. the watch points at, if it be going ; if not, set it a-going, and put the 

 indexes at what hour, min. and sec. you please. Let them go till sun-set, and 

 when the body of the sun is just half under the horizon, observe what hour, 

 min. and sec. the indexes of the watch point at, and note them also; then 

 reckon how many hours, &c. are passed by the watch between the observations ; 

 which is done by adding to the evening observation the hours, &c. that the 

 morning observation wanted of 12, or 24, in case the hour-hand has in the 

 mean time passed that hour once or twice ; otherwise, the diiFerence only gives 

 the time. Then take the half of that number, and add it to the hours, &c. of 

 the morning observation, and you will have the hours, &c. shown by the 

 watch, when the sun was in the south ; to which add the equation in the table 

 belonging to that day, and note the sum. Then, after some days do the same : 

 and if the hour of this last day be the same as noted before, your watch is well 

 adjusted ; but if it be more or less, the difference divided by the number of days 

 elapsed between the two observations will give the daily difference. 



Instead of the sun's rising and setting, you may take two equal altitudes of 

 the sun, before and afternoon, and having noted the time given by the watches 

 at the time of both the observations, proceed with it in the same manner as 

 above directed for observing the sun in the horizon. In either of which ways 

 there may be some error, caused by the sun's refraction, which is inconsiderable, 

 and therefore need not be noticed. 



6. By means of these watches, to find at sea the longitude of the place where 

 you are. 



Give to each of the watches a name or a mark, as A, B, C ; and before you 

 sail set them to the time observed by the sun in the place whence you are de- 

 parting, allowing for the equation of the day where you make the observation ; 

 which day you are to note, if the watches be not well adjusted ; otherwise it is 

 not necessary. Then, after being at sea, and desiring to know the longitude of 

 the place where you are, that is, how many degrees the meridian of that place 

 is more easterly, or westerly, than the meridian of that place where you set the 

 watches ; observe by the sun or stars, the time of the day, as precisely as is 

 possible, and note at the same time, to what hour, minutes and sec. the 

 watches point, which time, if the watches be not set to the right measure, is by 

 the known daily difference to be adjusted; adding the equation of the present 

 day, which gives the time of the day shown by the sun, at the place where the 

 watches were set : And if this time of the day be the same with that observed 

 where you are, then you are under the same meridian with the place where the 

 watches were set by the sun ; but if the time of the day observed where you are. 



