SCHUYLKILL PERMANENT BRIDGE. 47 



comport with long experience and observation, however sin- 

 gular it may appear. I endeavoured to prevail on the Board, 

 or the building committee, to cause the straight timber to be 

 hored through the heart ; that the air might pass, and the sap^ 

 there evaporate ; which, when confined, feculates, and soon- 

 er or later, rots all large timber. — Since this has not been 

 done, I am not displeased that the timber is splitting ; and, 

 through crevices, giving opportunities ot escape to this iates- 

 tine and deadly foe. Casing of such timber with lead^ t'ln^ 

 copper or xvood^ in Immediate contact, and thereby closing 

 the pores and preventing the emission of the feculating sap^ 

 I have endeavoured to shew to have been found, in most in- 

 stances, worse than fruitless ; because mischievous. Nothing 

 lias been proved so effectual, as covering the whole of a 

 frame, constructed of large timber, with a roof; and, at the 

 sides, excluding rain, without preventing an uninterrupted 

 circulation of air. The cover I propose is calculated to be 

 sufficiently strong for its own support ; but if tempests, un- 

 commonly violent assail it, the covering may blow away be- 

 fore the frame can be injured. The liglit sails may, by sud- 

 den squalls, be detached and yet the ship remain staunch. 

 These causes of apprehension, have, however, never struck 

 me with any force. Bare and improbable possibilities ought 

 not to be subjects of reasonable and sober, calculation. What 

 is certain far over-balances conjectural prognostics, — The 

 bridge if left uncovered, will most assuredly decay in ten or 

 twelve years. Experience is the best teacher in all cases. — 

 The modes of protection proposed in these drafts, have proof 

 to support them, dra^vn from long and respectable experi- 

 ment. Among others the Schaaffhaiisen bridge was a strong 

 instance. It had been by its cover, effectually preserved 

 from decay for thirty eight years^ and was perfectly sound, 

 at the time the French destroyed it. We have never heard 

 of its being injured by tempests, though in a situation much 

 exposed to tliem. In the eleventh report to the British Far- 



