(943)
Æquation of these days, taken out of the Table; And if the Aggregate doe Iust make 1z. hours, the Watch is set adjusted to the right measure; but if it differ, you are to divide the minutes and seconds of that difference by the number of the days between both the Observations, to get the daily difference, Let us suppose, this second Observation to have been made the 20. of March, viz, 8. days after the first, and finding, that the Middle of the Sun being seen in the Meridian in the same line
| h. | m. | sec. | |
| with the two threds, as before, the Watch points | 11 | 51 | 7 |
| The, Æquation of the 20 of March, by the Table, is | 0 | 10 | 40 |
| Which being added to the time, show'd by the Watch, gives | 12 | 1 | 47 |
If this had been Iust 12 hours, the Watch would have been well adjusted, but being 1. min. 47. sec. more than 12, it hath gone so much too fast in 8. days. And these 1 min. 47. sec. that is, 107. sec. being divided by 8, there comes. 13⅛ sec. for the difference of every 24. hours; which difference being known, if you want time, or have no mind to take the pains to adjust the Watch to its right measure (this being not necessary, since you may bring it thus on ship-board) note onely the daily difference, and regulate your self accordingly, as hath been mention'd. But if you will adjust it better, you must remove the less weight of the Pendulum a little downwards, which will make it goe slower; and then you must begin to observe anew by the Sun, as before, If it had gone too slow, you must have remov'd the mention'd weight somewhat upwards. And this is of that importance in the Ending out of Longitudes, that, if it be not observ'd, you may sometimes in the space of 3 months misreckon 7. degrees and more (yet without any fault in the Watches;) which under the Tropicks will amout to above 400 English miles.
Having shew'd, How the Watches may be adjusted at Land, or how their daily difference may be known; next shall follow, How the same may be done, when a Vessel rides at anchor, it being hardly fesible when she is under saile.
In the morning then, when the Sun is just half above the Horizon, note, what hour, min. and sec. the Watch points at,
