327. 3. The Neat’s-Foot-Oil prevents the Varnish from being sticky, or adhèsive: and may be put into the Linseed-Oil, at the same Time with the Lime, or burnt Umber. 327. 4. To make the above Varnish _transparent_, or _white_; use Mastic and Copal: to make it _brown_, use Seed or Shell-Lac, and _browner still_, use _pounded burnt_ Umber. 327. 5. _Rezin_, or _Chio Turpentine_ may be added, till the Varnish has obtained the desired _Thickness_. 327. 6. It must likewise be observed, that _Litharge_ rots the Silk: therefore Trials must be made without the Use of Litharge. 327. 7. The _longer_ the raw Linseed-Oil remains on the unslacked Lime, or Umber, the _sooner_ will the Oil dry, after it is used. If some Months; so much the better. Such Varnish will _set_, i.e. will not run, but keep its Place on the Silk, in four Hours. The Silk may then be turned, and varnished on the other Side. 328. +on gum mastic, sandarac, seed-lac, shell-lac, and copal.+ 328. 1. Gum _Mastic_ dissolves, _without pounding_, by adding a few Drops of Oil of Vitriol: so do Gum _Sandarac_, and Gum _Copal_, when finely pounded and sifted. 328. 2. Gum _Sandarac_, and Gum _Mastic_ are great Driers of themselves: and may be substituted for Litharge. 328. 3. The Mastic dissolved in the Oil of Vitriol, gives a _sweet_ Smell to the Varnish. 328. 4. Sandarac will soon grow _dusk_ in the Fire: it melts into a transparent Liquor. 328. 5. Sandarac, Seed-Lac, and Shell-Lac, must be finely pounded and sifted, before they are used. 329. The Author having examined different Kinds of varnished Silks, in different Places, does, from their Excellence, recommend those made by _Fawkner_, Umbrella-Maker, Alport-Street, Manchester; a Person wholly unknown to him, but from the Merit of the Work: which consists not only in the Varnish itself; but in the peculiar Method of _applying_ it, which the Author _is not at Liberty to make public_. _Fawkner_ can warrant his Silk _Air-tight_; _soft_ and _unadhesive_; durable, and _unalterable_ by that Excess of Heat and Cold, to which the Balloon is, at the same Time, subject; viz. _internally_, to the hot depredating and caustic Fumes, rising with the Gass: and _externally_, to the _Sun_, _Wet_, _Frost_, and _Drought_. CHAPTER LXIV. HINTS ON IMPROVEMENT OF THE MACHINERY. Section 330. In order to make Improvements of the Balloon still more rapid and general; the Society for the Encouragement of Arts, who have given no particular Encouragement, in Imitation of that at Lyons, to the much-wished-for Art of directing the Balloon;—might offer a Premium for different Inventions of a _propulsive Machinery_, the Models of which are to be made at the Expence of the Society, within a certain limited Sum: and, without condemning what cannot be known unless by repeated Trials,—give Encouragement for such Trials: the Models to remain with the Society for public Exhibition.