SCHUYLKILL PERMANENT BRIDGE. 35 



materials in proportion. Much of these may be saved, 

 in narrower frames. The painting or coating, with the 

 durable composition, in imitation of stone, which appears 

 on the exterior of the work, below the platform, (for 

 which a recipe is subjoined) may be done at a small 

 expence. Mineral paints are the worst, for coating ex- 

 posed to weather. The oil does not combine with the 

 mineral, as it does with absorbent earths : and being ex- 

 tracted by the sun, leaves the mineral particles without 

 adhesion, and they drop, or are washed away by rains, 

 dews, and moisture. All oils or fats, are known, chemi- 

 cally^ to be alike composed ; and are better or worse, as 

 they are or are not mixed with foreign matter. Linseed 

 oil may be had every where, and fish oil is common. 

 Ochres for colouring, (far preferable to minerals) abound 

 throughout the country i and only require judicious ex- 

 ploration for their discovery. Clarified turpentine is a 

 good substitute for oils ; but a mixture of both is best. 

 The Itssjbi'cing, to accelerate drying, the better. Though 

 inconvenient in some respects, the composition will be 

 more durable, the longer it is in drying; but care should 

 be taken, that it be not so thin as to run; or not retain the 

 sand and paint. Sea sanely or earth mixed with marine 

 salt, should be avoided, as being hostile to compositions 

 or cements; and particularly when calcareous substances 

 are combined. Some of the Delaware stone-cutters sand, 

 used with the Schuylkill bridge coating, was found to be 

 liable to this objection. We have daily before us proofs 

 of this fact in our plaistering; where the hair o^ salt hides 

 is used. Every moisture of the room, or atmosphere, 

 brings out stains and damp spots on our walls ; to which 

 papering will not adhere, as it does on other plaistering, 



