Lecture also because the Light is not so dense, there being VIII. much fewer Rays which can come to a certain Por Heat. tion of the Surface to heat it. Moreover the Sun being low in the Horizon all the Winter, the Beams pass thro' a much greater Quantity of Air, or are deeper immerfed in our Atmosphere in the Winter, than they are in Summer, when the Sun approaches nearer to our Vertex, and the Force of the Rays is broke by the Reflections on fo many Particles of Air: And the Difference is so very great, that when the Sun, is in the Horizon, we can look upon him without hurting our Eyes; but when he rifes higher, there is no enduring his Sight without blinding us. The Days BUT there is another very powerful Cause which longer than produces the Variety of Seafons; which is, that the the Nights, increase the longer any hard and folid Body is expofed to the Fire, the hotter it grows. Now in the Summer for fixteen Hours we are continually in the Sun's Heat, and we have only eight Hours in the Night to cool: The contrary of which happens in the Winter, and therefore it can be no Wonder, that there fhould be so great a Difference of Heat and Cold, in these two Seasons. SINCE the Power of the Sun is greatest when his Rays fall upon us most directly, and when the Days are longeft; it would feem that the greatest Heat ought to be when the Sun enters the Tropick of; for then the Sun comes nearest to our Vertex, and lieth longest upon us. But Experience fhews us, that we have the greatest Heat after that the Sun has left the Tropick; and the Seafon becomes warmest about the End of July, in the Dog-Days, when the Sun has paffed the Tropick, and is removed from it above a whole Sign. THAT we may give the true Cause of this Effect, it is to be observed, that the Action of the Sun, by which all Bodies are heated, is not tranfient, as its Illumination is, but permanent: So that a Body which has been once heated by the Sun, retains its Heat for fome Time after the Sun has gone when the Sun gone off it: So that the heating Particles which Lecture How from the Sun, and are absorbed by the heat- VIII. ed Body, do for a certain Time remain within it, and do therein raise a Warmth or Heat. But afterwards, when these Particles fly off, or lofe their Force, the Body begins to cool: And therefore, if the heating Particles, which are conftantly received, be more than they which fly away, or lose their Force, the Heat of the Body must Why the And this is our present Heat is not continually increase. Cafe: After the Sun has entered the Tropick, the greatest, Number of Particles which heat our Atmosphere is in the and Earth, does conftantly increase, there enter Summer Tropick. ing more in the Day-time than what we lose in the Night-time, and therefore our Heat muft grow greater. Let us fuppofe, for Example's fake, that there are a hundred heating Particles received in the Day-time in Sun-fbine, and the Night being much fhorter, there fhould fly off only fifty of them, other fifty ftill remaining there to excite · Heat: The next Day, the Sun acting with almost the fame Force, will impart another hundred Particles, of which no more than one half will fly away in the Night; fo that on the Beginning of the third Day, the Number of Particles exciting Heat, will be increased by one Hundred: And thus, while there are more Particles that excite Heat received in the Day-time, than what fly away in the Night, the Heat will conftantly grow stronger. But then, as the Days decrease, and the Action of the Sun becomes weaker, there will at last be more Particles that fly away in the Night-time, than what we receive in the Day-time; by which means the Heat of a Body will grow every Day lefs, and the Earth and Air will by Degrees cool. LECTURE TURE Lecture The Moon is an Atten dant on the Earth. It has va LECTURE IX. Of the Moon, its Phafes and Motion. F all the Bodies in the Heavens, if you except the Sun, the Moon appears to be the most splendid and fhining Globe, and does more particularly belong to our Earth, of which the is an infeparable Companion. And fhe does conftantly abide fo much in our Neighbourhood, that if fhe were looked at from the Sun, the could never be seen to depart from us by an Arch greater than ten Minutes. She therefore is tied to the Earth, and waits upon her as an Attendant, going along with the Earth round the Sun in the Space of a Year; but in the mean Time fhe has a proper Orbit of her own, which the describes round the Earth, in the Time of a Month. THE Primary Planets have the Sun, which they rious Shapes regard as a Center, for the Regulator of their Moand Phafes. tions; and fometimes they approach us nearly, at other times they move away to a great Diftance from us. But the Moon, like an Earthly Body, is kept in our Neighbourhood by a natural Propenfion or Gravity towards us; by the Means of which it is conftantly turned out of a rectilinear Course, and is obliged to perform its Revolution round about us, in the Space of 27 Days and seven Hours. The Moon puts on feveral Phafes and Appearances, and is always changing its Figure; and with the Multitude of her Forms, fhe has frequently puzled the Minds and Understandings. of thofe Philofophers, who have moft contem plated plated her: Sometimes fhe increases and grows bigger, then again the wanes, and diminishes, as it were, in old Age; fometimes fhe is bended into Horns, and then again the appears like a half Circle; at other times, the looks gibbous or hump-back'd, and immediately fhe affumes a full globular Face; and afterwards, by Degrees, the disappears, and lofes all her Luftre; fometimes fhe enlightens us the whole Night, at other times fhe does not appear 'till late at Night: And even in a total Eclipfe, fhe is frequently vifible, though with a very languid and pale Countenance: Sometimes fhe keeps in the Southern Region of the Heavens; at other times, the rises high, and vifits the Northern Hemisphere. All thefe Things were first found out by Endymion among the Greeks, who was the first among them who watched her Motions; and upon that Account, was fuppofed to have fallen in Love with her.. Lecture THE Moon, like the Earth, is a dark, opake, and The Moon is fpherical Body; and only shines with the borrowed a Spherical Light of the Sun: For it is the Sun who is the Opake Body. great Luminary in our Syftem, and who always illuftrates that Half of the Moon's Body, which is turned towards him; whilst the other Half, which is oppofite, is involved in Darkness: But the Face of the Moon, that can be seen by the Inhabitants of the Earth, is that which is turned toward the Earth: And therefore, according to the various Pofition of the Moon, in respect of the Sun and Earth, we do obferve different Illuminations and Degrees of Illustration; at one time a larger, at another a leffer Portion of the illuminated Surface is to be feen; fometimes there is no Part of it visible, and sometimes we observe the Whole, and fee the Moon with her full Face. But for the better Understanding of this Matter, we will explain it by a Figure. Let S represent the Sun, Plate V. T the Earth, RTS a Portion of the Earth's Fig. 1, 2. Orbit, which it defcribes in its Annual Course round Lecture round the Sun. Let ABCDEFGH be the Or IX. bit of the Moon, in which the turns round the Earth in the Space of a Month, from the, Weft towards the East. The true Motion of the Moon from Weft to Eaft. the Moon Our THIS Motion of the Moon is evident to Senfes; for if the Moon be observed to arrive at the Meridian any Night with a fixed Star, the next Night the will be 52 Minutes later in coming to the Meridian, or in Southing, than the Star; fhe having receded from the Star about 13 Degrees towards the Eaft. Join the Centers of the Sun and Moon, with the right Line SL; and through The Circle in the Center of the Moon, imagine a Plane MLN to pafs, to which the Line SL is perpendicular: bounding The Section of that Plane, with the Surface of the Light and Darkness. Moon, will produce the Circle which bounds Light and Darknefs in her, and feparates the inlightened Face, from the dark and obfcure Side. In the fame Manner, let the Centers of the Earth and Moon be joined by the right Line TL, which is perpendicular to a Plane PLO, paffing thro' the Center of the Moon; that Plane will make, on the Surface of the Moon, the Circle which diftinguifheth the vifible Hemifphere, or that which is towards us, from the invifible, which is turned from us; The Circle Which Circle may therefore be termed the Circle of Vifion. of Vifion. HENCE it is manifeft, that whenever the Moon is The Phafes in the Pofition A, in the Point of its Orbit oppoof the Moon fite to the Sun, that then the Circle bounding Light explained. and Darknefs, and the Circle of Vifion, do coin cide; and that all the illuminated Face of the Moon will be turned towards the Earth, and be vifible by its Inhabitants: And then the Moon is faid to be full, and the fhines all Night long; and in refpect to the Sun fhe is faid to be in Oppofition: For the Sun and Moon are feen in oppofite Parts of the Heavens, the one rifing when the other fets. When the Moon comes to B, the whole illuminated Disk MPN is not turned towards the Earth, there being a Part of it M P not to be seen by us; |