206. That a greater number of rays fall on the same place, when they come perpendicularly, than when they come obliquely on it, will appear by the Figure. For, let _AB_ be a certain number of the Sun’s rays falling on _CD_ (which, let us suppose to be _London_) on the 22d of _June_: but, on the 22d of _December_, the line _CD_, or _London_; has the oblique position _Cd_ to the same rays; and therefore scarce a third part of them falls upon it, or only those between _A_ and _e_; all the rest _eB_ being expended on the space _dP_, which is more than double the length of _CD_ or _Cd_. Besides, those parts which are once heated, retain the heat for some time; which, with the additional heat daily imparted, makes it continue to increase, though the Sun declines toward the south: and this is the reason why _July_ is hotter than _June_, although the Sun has withdrawn from the summer Tropic; as we find it is generally hotter at three in the afternoon, when the Sun has gone toward the west, than at noon when he is on the Meridian. Likewise, those places which are well cooled require time to be heated again; for the Sun’s rays do not heat even the surface of any body till they have been some time upon it. And therefore we find _January_ for the most part colder than _December_, although the Sun has withdrawn from the winter Tropic, and begins to dart his beams more perpendicularly upon us, when we have the position _CF_. An iron bar is not heated immediately upon being put into the fire, nor grows cold till some time after it has been taken out. CHAP. XI. _The Method of finding the Longitude by the Eclipses of Jupiter’s Satellites: The amazing Velocity of Light demonstrated by these Eclipses._ [Sidenote: First Meridian, and Longitude of places, what.] 207. Geographers arbitrarily choose to call the Meridian of some remarkable place _the first Meridian_. There they begin their reckoning; and just so many degrees and minutes as any other place is to the eastward or westward of that Meridian, so much east or west Longitude they say it has. A degree is the 360th part of a Circle, be it great or small; and a minute the 60th part of a degree. The _English_ Geographers reckon the Longitude from the Meridian of the Royal Observatory at _Greenwich_, and the _French_ from the Meridian of _Paris_. [Sidenote: PLATE V. Fig. II. Hour Circles. An hour of time equal to 15 degrees of motion.]