Of strange (jlasses, yji

Chap. XIV.

Of Burning-Glasses,

I Proceed re Burning-Glasses , which being opposed against the Sun beams, will■* kindle fire upon matter laid under them; In these also are the greatest secrets ofNature known. I (ball describe what is found out by Enclide, ‘Ptolemy, and Ar-chimedes - and I (ball add our own inventions, that the Readers may judge how farnew inventions exceed the old. Fircis kindled by reflection, retraction, and by aAmple and a compound Glass. I shall begin from a simple reflection, and from

A Concave-GUfs that fit all kindle fire behind it :

which few have observed. Know, that a Concave-glass will barn from its Middlepoint, wm;o the hcxagonal-side above the Glass, as far as a fourth part of its diame-ter . stem the hcxagonal-side, as far as the te-tragonal without the Glass, on the lowerpart of it: Wherefore cut off that part of thesemicircle, which is situate from a pentagonas far as a tetragon, as it were the band of thecircle 5 and * is being polished, and opposed•gainst the Sun , will cast fire far from it, be-*hinde it. I will say no more,because 1 said moreat large in my Opticks concerning this. So al-so we may

With a Concave Pillar or Pyramidal, kindle fire:but very slowly, with delay cnely,and in the Summer-Sun ; it kindles in the whol*line, and not in a point, but being extended by the point of accension of its circle*The fame will fall out by a Pyramidal Concave*

Meres

Chap. XV.

Of a Parabolical Settlor, that u of all Glasses the moss burning.

T Hac is called a Parabolical Section, that more forcibly farther off, ar.d in short-er time, will set matter on fire, that is opposite to it: it will melt Lead andTin : My friends related to me, that Gold and Silver also; but 1 have made them redhot. By which invention of o/irchtmedes , as appears by the testimony of Galen , andmany more, We read that he set the Reman Navy on fire, when MarceUfts besiegedSyracuse, his Country. Plutarch in the life of Pomfilttu faith, The fire that burntin "Diana's Temple, was lighted by this Glass, that is,by instruments that arc made ofthe side of right triangle, whose feet are equal: These made hollow, dofrom the cir-cumference respect one Centre. When therefore they are held against the Sun, sothat the beams kindled may be gathered from all parts, and be united in the Centre,and that they do sever the Air rarified, it soon sets on fire all fuel that is combu-stible opposed against it, by kindling first the lightest and driest parts; the beamsbeing as so maty fiery darts falling upon the Object, In a Concave spherical Glassthe beams meeting together, kindle fire in a fourth part of the diameter under theCentre, which are directed within the side of a Hexagon from the superficies of thecircie. But a Parabolical Section, is, wherein all the beams meet in one point fromall the parts ot its superficies. Cardarm teacheth how luch a Glass should be made.If we would kindle fire at a mile distance, we must describe a circle, whose diame-ter must be two miles longjand of this we must take such a part, that the roundness ofit way not lye hid,namely, a sixtieth parr,to which we must add a dimetienr,accordingto the altitude in one point, and upon the fixt diameter must we bring about partof the circle, which (hall describe the portion of a Sphere; which when We have po*

Odd 2 fished*