The whole is heated in an iron pot until fully liquified and thoroughly mixed. This cement is used almost in a state of ebullition, so that it cannot be handled without the precaution of continually dipping the hands in cold water. _Composition of cement used for the adjustment of the pieces of glass which touch each other._ 12 parts white lead. 1 do. minium or red lead. 5 do. boiled lintseed oil. The whole is pounded on an iron table by means of a flat mullet, like that used by painters (fig. 112), whose grinding surface is _a_ _b_, and _c_ the knob for the hand. This cement is applied liquid so as to offer no resistance to the close union of the pieces, which it is intended to unite. [Illustration: Fig. 112.] _Cement for filling up voids, and fixing the rings in the frames._ 12 parts white lead. 3 do. whiting. 1 do. minium. 4 do. boiled lintseed oil. This last composition differs from the former only in the introduction of whiting, which, while like minium it has a desiccative property, gives more body to the cement and prevents the formation of cracks. The oil is also decreased in quantity, as the cement must be used in a more compact state. The trituration of this cement is performed by means of a cylindric iron roller _a_ _b_, with a centre-spoke _c_ _d_ for the hand (fig. 113). [Illustration: Fig. 113.] It is essential for the production of good cement, that the mixture of the ingredients be complete. _Prices of the various parts of the Dioptric Apparatus._ The expense of the various parts of the Dioptric apparatus is as follows: Great lens of first order, L.58 (8 of which are required); pyramidal lens and mirror, L.14, 12s. (8 of which are required); catadioptric cupola, L.480; catadioptric rings below lenses, L.360; pannel of dioptric belt for fixed light of first order, L.56 (of which 8 are required for the whole circle); apparatus of fourth order, for a fixed light, for whole horizon, L.128; apparatus of sixth order, for whole horizon, L.44. The expense of the mechanical lamp of the first order with four wicks, as made for the Scotch Lighthouses by Mr JOHN MILNE of Edinburgh, is L.30. _Diagrams illustrative of the Table, Appendix, No. IV._ [Illustration: Fig. 114.] [Illustration: Fig. 115.] [Illustration: Fig. 116.] [Illustration: Fig. 117.] APPENDIX, No. IV. TABLE OF THE ELEMENTS OF CATADIOPTRIC ZONES SUITED FOR LIGHTS OF THE FIRST ORDER IN THE SYSTEM OF AUGUSTIN FRESNEL.