The Relation between these two Figures being well understood, it will follow from what precedes, That, _the sum of the _Sines_ of the Meridian Altitudes of the Sun in the two Tropicks, (and the like for any two opposite Parallels) being multiplied by the _Sine_ of the semidiurnal Arch, will give an _Area_ Analogous to the Curve Surface RIMSQDP; and thereto adding in Summer, or substracting in Winter, the Product of the length of the semidiurnal Arch, (taken according to _Van Ceulen_'s Numbers) into the difference of the above-said _Sines_ of the Meridian Altitude: The sum in one case, and difference in another, shall be as the Aggregate of all the _Sines_ of the Sun's Altitude, during his appearance above the Horizon; and consequently of all his Heat and Action on the Plain of the Horizon in the proposed Day_. And this may also be extended to the parts of the same Day; for if the aforesaid Sum of the _Sines_ of the Meridian Altitudes, be multiplied by half the Sum of the _Sines_ of the Sun's Horary distance from Noon, when the Times are before and after Noon; or by half their difference, when both are on the same side of the Meridian; and thereto in Summer, or therefrom in Winter, be added or substracted the Product of half the Arch answerable to the proposed interval of Time, into the difference of the _Sines_ of Meridian Altitudes, the Sum in one case and Difference in the other, shall be proportional to all the Action of the Sun during that space of time. I fore-see it will be Objected, that I take the _Radius_ of my Circle on which I erect my Perpendiculars always the same, whereas the Parallels of Declination are unequal; but to this I answer, That our said Circular Bases ought not to be Analogous to the Parallels, but to the Times of Revolution, which are equal in all of them. It may perhaps be useful to give an Example of the Computation of this Rule, which may seem difficult to some. Let the Solstitical Heat in ♋ and ♑ be required at _London_, _Lat._ 51° 32'. 380-2'8 _Co-Lat._ 23 -30 _Decl. ⨀_ ------ 61 -58 _Sinus_ = ,882674 14 -58 _Sinus_ = ,258257 ------ _Summa_ 1,140931 _Diff._ ,624417 _Diff. Ascen._ 3300-1'1. _Arch. Semid. æstiv._ 123-11. _Ar. Sem. hyb. 56-49. S._ ,638923 _Arch. æstiv. mensura_ 2,149955 _Arc. hyb. mensura_ ,991683 Then 1,140931 in ,836923, + 624417 in 2,149955 = 2,29734. And 1,140931 in 836929 - ,624417 in ,991638 = 33895. So that 2,29734 will be as the Tropical Summers Day Heat, and 0,33895 as the Action of the Sun in the Day of the Winter Solstice. After this manner I computed the following Table for every tenth Degree of Latitude, to the Æquinoctial and Tropical Sun, by which an Estimate may be made of the intermediate Degrees.