by the Birds through the Air after this manner, yet the Bird itself by being heavier than so much Air, would fall down, or at least fink leasurely downwards. But not to dwell too long upon Arguments only, we need only obferve for a Proof what has been faid, that great Birds, fuch as Oftritches, Storks and Swans (in which, by reafon of the flow Motion of their Wings, the fame may be clearly feen) in Flying, ftrike their Wings up and down, (or perpendicularly to the Horizon, as Mathematicians term it) whereby we find, that the Bird is at the fame time fupported and moves forwards in the Air.

Can we then perceive no Wisdom herein! That thefe Wings (Tab. XVII. Fig. 1.) AE and BF, of the flying Fowl B G A, are fomewhat hollow below, in order to take hold of the Air with fo much more Force and Power in ftriking them down; and above they are Convex, that in lifting 'em up they may meet with the lefsRefiftance from the Air; and fo that they mayn't lofe in the raifing of their Wings that which they gain'd inftriking them down, to keep them floating in the Air. But that which is here particularly to be obferved, is, that thefe Wings are not faften'd to the Body by their whole Breadth, but only at A and B, all the other Parts thereof being entirely loofe; whereby it happens, that (as may be feen in the Obfervations of Borelli, Prop. CLXXXIII, and CLXXXIV.) the faid Wings being raifed up, do only cut the Air upwards with the fharp Fore-part A E and BF, that they may meet with lefs Refiftance; but ftriking the Air downwards with a greater Swiftnefs, they defcribe with all their Points, Lines that are almoft circular, fuch as EIP and FVL.

But fince the wonderful Manner whereby a Bird cuts the Air with his Wings upwards and downwards, and moves them forwards at the famé defcribed

time with so great a Velocity, cannot fo eafily be described nor comprehended by Words; let us represent to our felves in Tab. XVII. Fig. 2. a Bird R S, as he floats in the Air, and extends both his Wings BE A and BCF, we may then fuppofe, that when thefe Wings are moved directly downwards, the Arms thereof, BC and BE, which being compofed of Bone, and therefore ftiff and hard enough, do defcribe two Circles whofe Planes make Right Angles with the Horizon, as in the foregoing Fig. 1. Tab. XVII. and fo caufe the whole Wing to follow that Motion, and to exert its Force with this perpendicular Blow upon the Air that lies under it, H G B E A.

Now, forafmuch as this Air when ftruck by the Concave Superficies of the faid Wing, makes a Refistance (as it happens when Women move their Fans through the Air) because it cannot recede quick enough: And moreover, as the Parts of the Air being compreffed by the Velocity of the Blow, do fenfibly endeavour to expand again, as we have fufficiently proved above in Contemplation XVII. about the Elafticity of the Air; and as appears plain enough from the rufhing Noife which Birds make by flying or ftirring their Wings; it will follow, that the Feathers E A O, by the faid Refiftance and Elafticity of the Air, will bend upwards,being made of a flexible Matter; and therefore when the Arms BE and BC, compofed of an inflexible Bone, purfue their way in ftriking downwards, the Ends of their Wings, A and E, will, by the bending of the Feathers upwards, be preffed towards each other.

From hence it is eafie to fee, that the Air being beaten downwards by the Wings, and by its Elafticity refifting upwards, the Bird is fupported in it by the repeated Reverberation at every Blow. And forafmuch as by the Flection upwards and VOL. II. downwards

Rr

downwards of the Feathers of the Wing,the Air receives the Blow obliquely in the Motion thereof, we may from thence give the reason why the Bird is thereby pushed forwards, and horizontally towards R, and fo is faid to perform the Action of Flying. So that the Beginning of the perpendicu-. lar ftriking upon the Air, does chiefly fupport the Bird, and the Continuation of the faid Blow does chiefly promote the Bird's progreffive Motion.

Perhaps this may be render'd more intelligible to fome, by fuppofing, as Borelli does, the Bird, R S, to be at reft, and without motion, and that it holds its Wings, BE A and CF, horizontal; and that by a Wind, HGO, blowing directly upwards against the said Wings, their Ends A and D being bent towards each other upon the Back of the Bird, the two Wings do thereby reprefent the Figure of a Wedge runnning obliquely into the Points AF. Now if both the Sides of this Wedge are preffed by the opening Air or Wind, every one knows that it must follow from thence, that it will be protruded towards its broadeft Part CBE, and fo carry with it the Bird RS, which is faften'd to it at O. Now thofe that understand Mechanicks, know well enough, that the fame Effect will be produced, whether the Air be moved upwards as a Wind, or the Wing downwards.

I wish I could here fubftitute any known Machine proper to fhew the true manner of the Action of the Wings, and to give a greater light to the Unexperienced, how the exactly circular ftriking down of the Arms or Bones that are in the Wings joined to the Flection of the Feathers upwards, can at the fame time support a Bird in the Air, and caufe him to fly forwards. But I must own I know of none my felf, nor find any such in others.

Something

Something like it, tho' very imperfect, occurs in the Sails of our Wind-Mills, as alfo in Ships that fail with a fide or half Wind; which however only fhews how the Wind blowing from one Point causes the Sails of a Mill or a Ship to move forwards towards one another: This happens in fome manner likewife to the Wings of a Bird when it flies, but does however by no means reprefent the true Manner of Flying.

Yet to fuggeft fomething that has a little more Analogy with the Motion of the Wings; let half a Sheet of Paper be fasten'd to a little Stick in the fame manner as the Colours are faften'd to an Enfign-Staff; the faid Stick is to represent the Arm or Bone of the Wing, and the flat Paper the Feathers,which must not hang down under the Stick, but be held up in the Air by it. Now if you move this Stick with your Hand in a direct circular Motion from above to below, and the same be done pretty fwiftly; you will fee, that the Paper is thereby moved, firft from beneath, upwards, and next from backwards, forwards; from whence one may form a rough Conception (fince the fame thing happens in each of the Wings on both fides of the Bird, by the ftriking down of the Arm) how the Bird moves upwards and forwards at the fame time; in which Flying confists.

SECT. XVI. The Wonderful Structure of the Wings.

Now whoever has attentively confider'd what has been faid, and understands what we have here faid about the Action of Flying, will fee, that in order to make a Bird fly, the Feathers of his Wings muft neceffarily be, Firft, Light, that they may not obftruct nor incumber him; Secondly, Flexible; and Thirdly, Stiff and Elaftical; that is, that be

ing bent, they may refume their natural State, by fpringing back of themselves.

Now let us Contemplate the fame, juft as we obferve them in Birds, and we shall find;

I. That the Quills to which the Feathers are faften'd are hollow, that they may be light, and nevertheless fliff and hard, as being compofed of a thin and horny Subftance.

II. The remaining or lower Part of the Quill muft not be inflexible, because in ftriking down of the Wing, it was neceffary that it fhould be capable of Inflection by the Refiftance of the Air, to the end, as we have faid before, that the two Wings might approach each other, in order to meet the Air obliquely, and protrude the Bird forwards. Now we find that this part of the Quill is filled with a Matter that is very flexible and light, and which feems to me to be found no where else but there, as indeed it is there only neceffary, for it does not feem reducible either to Bone, Flefh, Membrane or Tendon, or indeed to any kind of Parts that occur in thefe or other Animals. Now can any one pretend, that this is alfo to be afcribed to Chance, or Ignorant Caufes ?

III. Now it is not enough that thefe Quills fhould be flexible, for fo is a Rope too; but it is moreover requifite, that in the perpendicular Motion of the Wings, they fhould be ftiff and hard enough too, to act with fome Force upon the Air, and that being bent upwards by fuch Ading, they may in the lifting up of the Wing resume their former and concave Figure,

Now all this concurs in the Structure of the Quill; for in the external Circular Part thereof it is cover'd with a Bark, which is in fome measure

hard,