Page:Philosophical Transactions - Volume 004.djvu/54

 By the help of this Table you will always know, what a Clock it is by the Sun precisely, and consequently, whether the Watches have been set to the right measure of the Mean day, Or no; using the Table, as follows.

When you first set your Watch by the Sun, you are to subduct from the time observed by the Sun, the Æquation adjoyned to that day of the Month in the Table, and to set the Watches to the remaining hours, minuts and seconds, that is, ills! Watches are to be set so much flower, than the time of the Sun, as (in the Table) is the Æquation of that day; so that the Æquation of the Day, added to the time of the Clock, is the true time by the Sun. And when after some days, you desire to know by the Watch the time by the Sun, you are to add to the time, shew'd by the Watch, the Æquation of that day; and the Aggregate shall be the time by the Sun, if the Watch hath been perfectly well adjusted after the measure of the Mean days; for the doing of which, this will be a Convenient way;

Draw a Meridian line upon a floor (the manner of doing which is sufficiently known; and note, that the utmost exactness herein is not necessary:) and then hang two plummets, each by a small thred or wire, directly over the said Meridian, at the distance of some 2. foot or more one from the other, as the smalness of the thred will admit. When the middle of the Sun the Eye being placed so, as to bring both the threds into one line) appears to be in the same line exactly (for the better and more secure discerning whereof, you must be furnish't with a glass of a dark colour, or somewhat black: with the smoak of a Candle,) you are then immediately to set the Watch, not precisely to the hour of 12. but by so much less, as is the Æquation of that day by the Table. E. g. If it were the 12 of March, the Æquation of that day being by the Table, 8 min. 3 sec; these are to be subducted from 12. hours, and the remainder will be 11. hours, 51. min. 57. sec; to which hours, minutes and seconds you are to set the Index of the Watch respectively: Then after some days you are to observe again in the same manner, and, likewise to note the hour, min, and sec, of the Watch; to which you are to add the Rh