Another collection of philosophical conferences of the French virtuosi upon questions of all sorts for the improving of natural knowledg made in the assembly of the Beaux Esprits at Paris by the most ingenious persons of that nation / render'd into English by G. Havers, Gent. & J. Davies ..., Gent.

CONFERENCE CLIII. Why 'tis colder at Day-break than at any other time of the Night or Day. (Book 153)

IF Cold be a real quality, then the greater distance there hap∣pens to be between it and the Source of Heat and Light, (the Sun) the greater must the Cold be. And if it be only a priva∣tion of Heat, then mid-night is darker then either the Evening or Morning, because oppos'd directly to the Light of the Sun, it may seem that the Cold ought be greater likewise at that time,

because the same is opposite to Noon, when the Sun's Heat is greatest; yet the cool of the Morning argues the contrary, being so ordinary that it fore-tells Day-break more certainly, than the crowing of the Cock. Unless you will attribute the cause to this, that at Morning before Sun-rise 'tis longer since the Sun in∣lightned the Horizon, than at mid-night; at which time the Air and other Elements, still retain some of the preceding Day's Heat, which tempering the Cold, occasion'd by the Sun's ab∣sence, renders the same less perceptible during the thickness of the Nocturnal Air, less subtile than that of the Day; when the Light coming to dissipate those Clouds, subtilizes the Air by its insinuating beams; whence the Cold thereof more easily in∣sinuates into our Pores by the help of that weak Light, which is not strong enough to heat the Air. Just as Vinegar, though hot and biting of its own Nature, yet mix'd with much water, cooles the part whereunto 'tis apply'd more than water alone doth.

The Second said, That possibly the comparison of the Heat of our beds, (out of which we arise in the Morning) with the cold of the outward Air, makes us guilty of a mistake; unless you had rather refer this Effect to the Oblique Aspect where-with the other Celestial Bodies of our Hemisphere are regarded by the Sun at his rising. For at mid-night when he is directly under the Horizon, the little bulk of the Earth hinders not, but he di∣rectly darts his Rayes upon those Stars which are above us, the Pyramid of the Earth's shadow not passing beyond the Moon; so that then the vast and incredible magnitude of all those Cele∣stial Bodies perpendicularly reflects upon us the Heat and Light of the Sun, which thus reflected may calefie the Air, as the Sun doth in the same posture; but not at all at Sun-rise in their Ob∣lique Aspects: Whence though the Sun be nearer us in Winter, yet he warms us less. If it be excepted, that the Evening, when the same Oblique Aspects return, is not so cold as the Night; 'tis answer'd, that this difference proceeds from the Heat of the fore∣going Day, remaining in the Earth, Water, and Air, which con∣serve the same, till by the absence of the Sun the supervening Night wholly dissipate them.

The Third said, That the Matutinal coolness proceeded from the approaching Suns driving the Clouds before him, which agitation raiseth a wind (as there is always one at day-break) whereby the same coolness is effected in the Air that a Fan cau∣seth to a Lady. For all things here below having their motion from East to West, 'tis reasonable that the Air be so mov'd too, and acquire the consequent of its agitation, namely, coldness. That all things come from the East, sundry instances manifest; Mankind was from thence diffus'd into the other Quarters of the World; Rivers run generally Eastward; And the greater speed of Navigation from East to West, than contrarily, argnes the Sea to have the same motion; as is chiefly observ'd under the

Equinoctial, the greatness of which Circle renders that motion more manifest. This rule the Winds keep, when not diverted to a contrary course by Exhalations: And as for the Heavens, experience shews us, that their ordinary and best-known course is from East to West: So that 'tis no wonder if they hurry the neighbouring Air with them, and by a Mathematical contact and natural consecution, all the other Elements. I speak not of Sciences, Arts, Policy, and other things which the more curious may find to have been deriv'd from the East. It suffices that the Sun taking this road drives the Air befor him; the wind pro∣ceeding from which motion, causeth the coolness we feel chiefly at day-break; when the vapours between us and the Sun being by his heat violently driven (as the water of the Aeolipila is turn'd into wind and driven forth by the subjacent fire) the coolness is more unacceptable, in that it succeeds and multiplies (instead of diminishing) that of the night, as the diurnal heat in likelihood ought to do.

The Fourth said, He attributed the increase of cold at day-break to the ordinary action of all natural Agents, which is strongest when they arrive at the period or utmost point of their declination. So a Candle just upon extinguishing casts forth a smarter flame; the violence of a Disease is greatest at its crisis, when 'tis towards ending; a Stone moves swiftest as it approach∣eth its Centre. And to compare the Year to the Day, the cold is commonly greater and more insupportable in February, the last Moneth of Winter, than in the beginning thereof; though in reason it might seem rather to be so at the end of December, when the Sun is further from us, and that the custom of the two first months cold should render this last more tolerable; as on the contrary, the heat is greater also in the dog-days, and after∣wards, than at the Summer Solstice, when the Sun is elevated highest above our heads. So also in Summer 'tis hotter two hours after noon, than at noon it self; not so much through any disposition already received in the Air and Earth, as by reason of that Rule, That Natural Actions are stronger at the end than the beginning; whereas violent actions, as the motion of a Stone upwards, is swifter in the beginning than the end.

The Fifth referr'd this effect to the Antiperistasis of heat and cold. For as fire seems more scorching upon the approach of a great frost, so by a contrary reason cold must become more ve∣hement at the approach of the Sun's heat. Moreover, the like combat is observ'd between the thickness of the darkness of the night, and the rarity of the day, when the Sun's light rendring the illuminated Air more subtle, what was gross in the dark Air cannot be expell'd in an instant without some conflict and moti∣on of the part condens'd by darkness with the rarefi'd by light; from which agitation ariseth a wind commonly at day-break, which is probably the cause of the cold at that time. Now of that tenebrous part condens'd is made the Dew and Frost in our

Climate, and the Manna in Southern Countries; as the cold which we feel redoubled in Winter in the space between a neigh∣bouring fire (but out of its Sphere of Activity) and the rest of the Air, is a familiar example of this Antiperistasis of heat and cold redoubled upon the approach one of the other. For as 'tis much colder then elsewhere, between that fire which is too distant to warm us, and the Air left in its natural frigidity; so at day-break our Air being too far off from the Sun to be heated by it, augments its coldness upon his approach.

The Sixth said, Air hath no natural quality but supream hu∣midity, whereby 'tis supple, movable, and pliant; heat and cold being impress'd upon it by outward agents: Otherwise be∣ing the general medium and mediator of motions local, natural, vital and animal (for the Spirits are of an aerious nature) and the Factor of all Agents by whose intervention they communicate their influences, it would act against the qualities impress'd up∣on it, sometimes hot and sometimes cold, and destroy them by its own. Which indeed its humidity doth, but to the profit of animated bodies; dryness being their enemy. Hence cold and dry Saturn hath under him, hot and moist Jupiter, who tempers his hurtfulness and sutes him to living things. Now the Sun having at the declining of the day rais'd many aqueous, and consequently supreamly cold, and the heat whereby they were rais'd abandoning them upon his absence, the natural cold of those vapours becomes predominant, and returns them by de∣grees into their first state. Which refrigerating the Air makes the night the colder, the further the vapours are from their extraneous heat, that is to say, the nearer day ap∣proacheth.