Of flrange (j lasses*

M

M,

K,

line B D; the first so F, the second in G, the thi/in H > thence I take my Section to be perfectedLKM, and lent the line K A into four parts, andthorow those points I draw parallel lines to LM.LecBH be the ueerest to the top of the ParabolicalSection, the second BG that 'oHows next, andthe third B F next to that, and after (ball be L M.Thence by the lines LFGHA, draw a crookedline, and do the fame on the other part so far as M,and that shall be the line fought for, to mal e the Pa-rabolical Section , and from that must be made theGlass, as I shall sliew.

Chap. XVI.

Herv 4 Parabolic al Section may be described , that may burn obliquely , and at a very

great distance.

I Have described a Parabolical Section, which might be made by rule and compass,because we may use it at a short distance; but in greater distance we mast proceedby numbers r as for forty or for sixty foot, and not much more, lest the Glass shouldbe made of an unusual magnitude. The forefaid Glass burns between it and theSun j and if the Sun be not as you desire it, the operation is lost r so also by an obliqueGlass, that is between the Sun and the combustible matter,or over against it.Whenceaccording to the situation you may use them all, nimely, wherein they answer yourexpectation ; and especially when the Sun is in the Meridian , they burn with morevehemency. This I must tell you,that you may not be deceived ;for when you erre,you commonly draw others into error with you. A Parabolical Glass made fromthe top, if the Section shall be from the top, if we would burn far, the Glass willbe plain ; and that it may havefome crookedness, it will be wonderful great. Andif the Section be about the basis, that will be worst of all; for from the least distance,it will he almost flat: wherefore that we may have it with some crookedness, wemust take a line about the neck of the Section, not the head, nor the feet. Where-fore being to make a Glass of a Parabolical Section, about the neck of the Section,where the greatest crookedness of the Parabolical Section is made, and that mayburn far from its superficies, to twenty foot distance ; Let the line AB be th e. sinusverfm eighteen foot long: from the point A, I raise a line to right Angles with A B,which shall be the line by which,the fourth part whereof is A B: cut A B so C,and letit be two foot,and C B sixteen footsl multiply twice seventy two,and that makes onehundred forty and fours the square root of this is twelve; wherefore the line ere-cted perpendicularly from the point C, unto the circumference of the ParabolicalSection , will be DI of twelve foot, wherefore CI will be the line appointed: joyn

I B, and the Radius that must burn, willbe in the point B that was fought for:Wherefore the ray of the Sun, that ise.quidistant to the (intu verfm H I, is refle -cted by IB inB; the Latitude whereofwill be about twenty foot: for the lineIC of twelve foot,multiplied into itself,will make one hundred forty and four;and C B is sixteen foot, which multipliedinto it self, makes two hundred fifty andsix; adde these together, and they makefour hundred: the square rootcf it is twenty foot, thus. Wherefore I atct resolvedto take the part of the Glass , intercepted between the points I and F, and I seektwo thirds of one foot, from C toward B, and I divide one foot into thirty parts,that the crookedness may be taken more precisely ; and let C G be twenty parts of

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