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thefe Properties in that, and the fame Properties in common Air, and the means by which he investigated that Proportion.
142. First. That the medium which propagates Sound is the common air; and confequently, that the finer and thinner elastic air or æther beyond the atmosphere is the medium which conveys or propagates light from the heavenly bodies.
143. Secondly. That sound is observed to move the space of an hundred miles, in much about the same space of time, that light moves from the Sun to the Earth, which distance Sir Ifaac computed at 70 millions of miles; therea fore as 1oo is to i, so is 70000ooo of miles, to 700000 miles : From thence he infers, that not only the different velocities of light and sound, are in the proportion of 700000 to 1, but the density, rarity, and elasticity, are in the same proportion also. See his own words. 144.
* Şounds move about 1140 englich • feet in a second minute of time, and in seven • or eight ininutes of time they move about
one hundred english miles. Light moves « from the Sun to us in about seven or eight
minutes of time, which distance is about
70000000 english miles, supposing the ho• rizontal parallax of the sun to be about 12". • And the vibrations or pulses of this medium,
that they may cause the alternate fits of easy • transmission and easy reflexion, must be swifter than light, and by consequence, above
700000 times fwifter than founds and
therefore the elastic force of this medium, ' in proportion to its density, must be above
700000 multiplied by 700000, that is, above
490000000000 times greater than the elastie * force of the Air is in proportion to its den,
sity. For the velocities of the pulses of elastic * mediums are in a subduplicate ratio of the • Elasticities, and the Rarities of the Mediums taken together. Optics. Quære 21.
145. The latter part of the above Quære, and the next following it, containing what may serve to illustrate the foremention'dreasoning concerning the rarity of the æther or pure air, as also its expansive force, are as follow :
146. As Attraction is stronger in small Magnets than in great ones, in proportion to
their bulk ; and Gravity is greater in the fur• faces of small Planets than in those of
great ones, in proportion to their bulk ; and small
bodies are agitated much more by electric & attraction than great ones : So the smallness
of the rays of light may contribute very much to the power of the agent by which they are
refracted. And so if any one should suppose, " that æther (like our air ) may contain parti, • cles which endeavour to recede from one • another (for I do not know what this Æther
is) and that its particles are exceedingly • smaller than those of air, or even than those ? of light: The exceeding smallness of its par: ticles. may contribate to the greatness of the
force by which those particles may recede • from one another, and thereby make that • '
medium exceedingly more rare and elastic * than air, and by consequence exceedingly less • able to resist the motions of projectiles, and
exceedingly more able to press upon gross bodies, by endeavouring to expand itself.
147. • Qu. 22. May not Planets and Comets, and all gross bodies, perform their mo« tions more freely and with less resistance in
this ætherial medium than in any fluid, which fills all space adequately, without leaving any pores, and by consequence is much denser than * Quicksilver or Gold + ? And may not its re• fiftance be so small as to be inconsiderable ? . For instance ; If this æther (for fo I will call
it) should be suppos'd 700000 times more • elastic than our air, and above 700000 times
niore rare, its resistance would be above • 600000000 times less than that of water; i and so small a resistance would scarce make any
sensible alteration in the motions of the « Planets in ten thousand
any one I would ask how a medium can be so rare, let
him tell me how the air in the upper parts • of the atmosphere can be above an hundred • thousand thousand times' (i. e. an hundred millions of times) ' rarer than gold? Let him • also tell me how an electric body can by • friction emit an exhalation fo rare and subtile,
t An Allusion to the Fluid of Des Cartes.
• and yet so potent, as by its emission to cause
no sensible diminution of the weight of the - electric body, and to be expanded through a ! sphere, whose diameter is above two feet, • and yet to be able to agitate and carry up ' leaf copper or leaf gold, at the distance of * above a foot from the electric body? And · how the effluvia, of à magnet can be so rare • and subtile, as to pass through a plate of glass without any resistance or diminution of their
force, and yet so potent, as to turn a magne • tic needle beyond the glass?' And inay not I add? Let him tell him how the electrical effluvia can be so rare and subtile, as to pass freely through a glafs receiver, and yet so potent, as to act on a downy feather, with no less force and activity, than what we perceive to be acquired by the magnetic needle, from the best of loaditones? ment may be made thus. ..
148. Let a downy feather be faften'd to the top of a small wire or stick, on a foot or stand, and cover'd with a tall glass receiver ; then if a glass tube be rubbed within a foot of the receiver, the feather (notwithstanding the intera position of the glass) will follow the motion of the hand rubbing the tube.
149. In all human probability no expedient could have been thought of, nor any experimnent invented to have proved the exceeding sarity and elasticity of æther, besides those prin ciples on which that fagacious enquirer so happi
ly proceeded : Possibly they might for ever have remain’d in oblivion. For altho' the general Phänomena arising from the natural effects have been observ'd by many ; yet we do not hear of any who ever represented it as poffess’d of those properties in such an eininent degree, Or that ever thought of any expedient whereby to compare it with the common air, and compute the proportion of the difference between those properties in æther and the fame
properties in air.
From the effects then which Sir Isaac had been mentioning, his conclusions were (as
before observ'd) that æther muft necessarily be - at least 700000 times more rare than the com
mon air, and yet 700000 times more elastic. Properties, which altho' they feem incompatible with each other, are yet verified by the electrical fuid, in which those properties are observ'd to exist in a very surprising degree, although the preciseness of the proportion may not be easily determin'd: Probably, as before ob served, there is no other means of making any tolerable computation than those which are there laid down.
151. If those natural properties fo obvious in the electrical Aluid are heedlesly pass'd over and disregarded, the experiments seem irreconcileable with each other, and consequently inexplicable ; but when those properties are duly examin'd and consider'd, they become such a key as to render the most abstruse of them plain