344 PHILOSOPHICAL TRANSACTIONS. [anNO 166Q, 



altered and enlarged, by two eminent members of tbe Royal Society, as 

 follows : 



1 . Those who intend to make use of pendulum watches at sea, must have two 

 of them at least, that, if one of them should by accident or neglect, or long 

 use, happen to fail, there may still remain one for use. 



2. They must procure good information of the nature of the inward parts of 

 the watches, the manner of winding them up, and how to set the indexes or 

 hands having the hours, minutes, seconds, &c. 



3. The watches on ship-board are to be hung in a close place, where they 

 may be most free from moisture or dust, and out of danger of being disordered 

 by knocking or touching. 



4. Before the watches be brought on ship-board, they should be adjusted to 

 keep mean time, the use of them being then most easy. Yet if time or con- 

 veniency for this purpose be wanting, they may notwithstanding be used at sea 

 with equal certainty, provided you know how much they go too fast or too slow 

 in 24 hours. 



5. To adjust or regulate the watches to keep equal or mean time: take 

 notice that the sun or the earth passes the 1 2 signs, or makes an entire revolu- 

 tion in the ecliptic, in nearly 365 days 5 hours 49 min. and that those days 

 reckoned from noon to noon are of different lengths. Now between the 

 longest and the shortest of those days, a day may be taken of such a length as 

 365 such days 5 hours, &c. the same numbers as before, may be equal to that 

 revolution : And this is called the equal or mean day, according to which the 

 watches are to be adjusted; and therefore the hour or minute shown by the 

 watches, though they be perfectly just and equal, must needs differ almost con- 

 tinually from those that are shown by the sun, or are reckoned according to his 

 motion. But this difference is regular, and is otherwise called the equation, a 

 table of which for the year you must be provided with.* 



By this table you will always know what o'clock it is by the sun precisely, 

 and consequently whether the watches have been set to the right measure of the 

 mean day or not, using the table, as follows : When you first set your watch 

 by the sun, deduct from the time observed by the sun the equation adjoined to 

 that day of the month in the table, and set the watches to the remaining hours, 

 minutes and seconds, that is, the watches are to be set so much slower than 

 the time of the sun, as is the equation of that day ; so that the equation of the 



* A general table of the equation of time, for every day in the year, was here inserted in the ori- 

 ginal ; but, being very erroneous in every number, it is omitted, and witliout supplying its place 

 tvith others, because every year requires a different set of numbers, for each day of the years. 



