1819."! 



THE CIVIL ENGINEER AND ARCHITECT'S JOURNAL. 



333 



In this expression (d), which represents the deflection, has dis- 

 appeared, and as {w') is a constant quantity for the same beam, 

 representing the weight that produces a deflection equal to the 

 unit of measure, it follows that P is the same with every weight 

 and every degree of deflection within the elastic limits. 



This result seems at first view to be contrary to fact; it would 

 appear that if the weight is increased, the horizontal strain should 

 be increased in the same proportion; but when it is remembered 

 that the deflection increases with the weight, and that the former 

 diminishes the value of P in precisely the same proportion that 

 the latter increases it, the difliculty vanislies, and the reason why 

 P should be constant for the same beam becomes obvious. 



The practical importance of this result is very great, as it fur- 

 nishes the means of obtaining a formula which will give at once 

 the extreme limit of the resistance to flexure, or the weight which, 

 applied to a post, will cause it to yield by bending. 



As the formulas used by Tredgold are calculated for a deflection 

 of ^th of an inch to 1 foot, or ^^th of the lengtli, the weight 



which would cause a deflection of 1 would hew 1 1 -r- 77,- ) =: - — ; — > 



\ 480/ i 



and by substituting this value for m' in the equation P := -^ w'l, 

 we find P — SO w — A. 



But from the ordinary formula for the stifi'ness of a beam sup- 

 ported at tlie ends we have 



w = —,. Therefore P — — rr- = B. 



d' cl- 



The expression P =: 90 ro shows that the extreme limit of the 



Btrength of any post whatever, of any length, breadth, or depth, 



oi' of any kind of material, is 90 times tlie weight which Ciiuses 



a deflection of -j j^th of the length. 



90 hd^ 

 The second expression, P = — tj— , will give the value of P 



directly, without first knowing the weight required to cause a 

 given deflection in a horizontally-supported beam. In this expres- 

 sion, b :=:; breadth in inches, d =: deptli or least dimension in 

 inches, / = length in feet, and c = a constant to be determined 

 by experiment for each species of material. 

 The value of c for white pine is -01. By substituting this value, 



9000 bd'^ ^ 



we find P = ji — , a remarkably simple formula, which give 



the extreme limit of the resistance to flexure of a white pine post. 



The same expression may be used to determine the constant 



used in the ordinary formula for the stifl^ness of beams. For tliis 



90 bd'' 

 purpose let the equation P = — -3— be transposed, which wil 



d- 



give e 



Find P by applying a string to a flexible strip o 



90b(P 



"W 



the material to be experimented upon, in the manner of a chord to 

 an arc, and ascertain the tension on the chord with an accurate 

 spring balance. It will be found that, whether the strip be bent 

 much or little, the tension on the chord, as shown by the spring 

 balance, will be constant; and this tension, in pounds, substituted 

 for P, will give the value of c without requiring, as is necessary 

 with other formula?, an observation of the deflection. 



Experiments made upon these principles with strips of white 

 pine, yellow pine, and white oak, 5 feet long, 1^ inch wide, and 

 ^-incli deep, give the following results: — 



The observed tensions were — 



AVhite Pine, 7+ lb. value of c 

 Yellow Pine, 6| lb. " 



A\lute Oak, 6| lb. " 



As the stifi'ness is inversely as their constants, it follows that 

 wliite pine is stiffer than yellow pine or oak. The experiments of 

 Tredgold give similar results. 



•0097 

 •0108 

 •OlO-l. 



New Kind of Gutta-Percha. — It is stated in a recent number of the 

 Amsterdam's Handelsblad, that there is every reason to believe that at 

 Palembang, in the eastern possessions of the Dutch, in the interior of the 

 country, gutta-percha, or getah perija, will be collected in great abundance. 

 It would appear also that another substance has been discoven-d, called 

 getah-malah-buay, which is also obtaiued in great abundance, by merely 

 bleeding the trees; and which, although not susceptible of the same ex- 

 tended applications as gutta-percha, may, nevertheless, when mixed with 

 this latter, prove of very great utility. 



DRY ROT. 



An accnunt of an extraordinary instance of the Rapid Dccnij of 

 Timber from Dry Rut, jvhich occurred in the Church of the Holy 

 Trinity, at Cork. By Sir Thomas Deanb. — (Paper read at the 

 Institution of Civil Engineers of Ireland.) 



The parish Church of the Holy Trinity in Cork, in the year 

 1827, having been found to be in a bad state of repair, and quite 

 deformed from bad and unequal foundations, the parishioners 

 resolved on building a new church; but, through want of funds, 

 not being able to carry their design into execution, an extensive 

 repair was decided on. The tower was taken down, and one side 

 wall, and the end of the church was rebuilt. 



This church is 100 feet long, by 50 feet wide, divided into a nave 

 and aisles by double tiers of columns, the lower tier being of solid 

 timber, supporting galleries, and resting upon rude rubble stone 

 piers, in the vaults below, the upper tier being of built timber 

 columns supporting the roof. It is necessary to describe the 

 building, in order to show that from retaining a part of the old 

 timber work, the evil of dry rot emanated. 



For years there had not been anything intervening between a 

 great ])art of the body of the old church and the burial vaults 

 beneath, except a timber floor, and though the interior was spa- 

 cious, and even handsome, this abomination long continued. 



Immediately under the floor of the church, and open to the 

 vaults, longitudinal beams of Irish oak, of from 13 to It inches 

 square, had been placed, resting on piers, and forming supports 

 for the joists. Though these oak beams were decayed for an inch 

 deep at their surfaces, suflicient of the timber (as it was thought) 

 remained sound, and it was decided that neither they, nor tlie 

 piers upon which they rested, should be removed. The vaults were 

 arched over, memel joists, 6 inches by 4 inches, were placed on the 

 vaulting, and connected with the old oak beams which rested on 

 the piers; tlie floors were removed, the old pews replaced, new 

 columns, coated with scagliola, were erected over the galleries, the 

 old ones in the lower tier retained; and the whole repairs having 

 been thus completed, the diurch was re-opened for divine serx'ice, 

 in April, 1829. 



In November, 1830 (but eighteen months afterwards), the con- 

 gregation was annoyed by an unpleasant smell, which, on ex- 

 amination, was found to proceed from dry rot of the most alarm- 

 ing nature. 



On opening the floors under the pews, a most extraordinary 

 appearance presented itself. There were flat fungi of immense 

 size and thickness, some so large as almost to occupy a space equal 

 to the size of a pew, and from 1 to 3 inches thick. In other places 

 fungi appeared growing with the ordinary dry rot, some of an 

 unusual shape, in form like a convolvulus, with stems of from a 

 5 to ;i an inch in diameter. AV'hen first exposed, tlie whole was 

 of a beautiful bufl' colour, and emitted the usual smell of the dry 

 rot fungus. 



^Vhatexer may have been the surprise at the rapid growth of 

 the plant, its action on the best memel timber was a source of 

 greater astonishment. I took up, with nearly as much ease as I 

 would a walking cane, that which, eighteen months before, was a 

 sound piece of timber (one of the joists), from 12 to l* feet 

 long, 6 inches by l inches scantling; the form of the timber re- 

 mained as it came from the saw, but its strength and weight were 

 gone. The timber of the joists and floor over the new brick vault- 

 ing was completely affected by the dry rot, which was rapidly 

 spreading to tiie lower part of the columns under the galleries, so 

 that at the rate the infection proceeded, the total destruction of 

 the building would soon have been efl'ected. 



During a great part of the time occupied in the repairs of the 

 church, the weather was very rainy. The arches of the vaults 

 having been turned before the roof was slated, the rain water 

 saturated the partly decayed oak beams, before described. The 

 flooring and joists, composed of fresh timber, were liud on the 

 vaulting before it was dry, coming in contact at the same time 

 with the old oak timber, which was abundantly supplied with the 

 seeds of decay, stimulated by moisture, the bad atmosphere of an 

 ill-contrived burial place, and afterwards by heat from the stoves 

 constantly in use. All these circumstances account satisfactorily 

 to my mind, for the extraordinary and rapid growth of the fungi. 



The large sum of 4,000/. having been so lately expended on the 

 church, caused gi-eat anxiety to the parishioners. The opinions of 

 the most experienced professional men were taken, and all agreed 

 that the first effort should be to cut oif the communication with 

 the galleries, the disease having already extended 3 feet up\vards 

 on tlie lower C(dumus. 



The new brick vaulting was found penetrated by the fungi, im- 



