J839.] 



THE CIVIL ENGINEER AND ARCHITECT'S JOURNAL. 



333 



many other buildings in that vicinity, constructed of the limestone of 

 Ham Hill, are in excellent condition. In Salisbury Cathedral, built 

 of stone from Chilmark, we have evidence of the general durability 

 of siliciferous limestone ; for, although the west front has somewhat 

 yielded to the effects of the atmosphere, the excellent condition of the 

 building generally is most striking. 



In the public buildings of Oxford we have a marked instance both 

 of decomposition and durability in the materials employed ; for whilst 

 a shelly oolite similar to that of Taynton, whicli is employed in the 

 more ancient parts of the Cathedral, in Merton College, Chapel, &c., 

 and commonly for the plinths, string courses, and exposed portions of 

 tlic other edifices in that city, is generally in a good state of preserva- 

 tion, a calcareous stone from Heddington, employed in nearly the whole 

 of the colleges, churches, and oth.er public buildings, is in sucli a de- 

 plorable state of decay, that in many instances all traces of architec- 

 tural decoration have disajjpeared, and the ashler itself is in many 

 places deeply disintegrated. 



In Spofforth Castle we have a striking example of the unequal de- 

 composition of two materials, a magnesian limestone and a sandstone ; 

 the former employed in the decorative parts, and the latter for the 

 ashler or plain facing of the walls. Although the magnesian limestone 

 has been equally exposed with the sandstone to the decomposing 

 elt'ects of the atmosphere, it has remained as perfect in form as when 

 first employed, while the sandstone has sutfered considerably from the 

 effects of decomposition. 



In Chepstow Castle may be observed a magnesian limestone in fine 

 preservation, and a red sandstone in an advanced state of decomposi- 

 tion, both having been exposed to the same conditions as parts of the 

 same archways ; and in Bristol Cathedral we have a curious instance 

 of the effects arising from the intermixture of very different materials, 

 a yellow limestone and a red sandstone, which have been indiscrimi- 

 nately employed both for the plain and decorative parts of the build- 

 ing. Not only is the appearance in this case unsightly, but the archi- 

 tectiu'al effect of the edifice is also much impaired bj' the unequal 

 decomposition of the two materials, the limestone having suffered 

 much less from decay than the sandstones. 



Judging, therefore, from the evidence afforded by buildings of various 

 dates, there are many varieties of sandstone and limestone employed 

 for building piu^jjoses which successfully resist the destructive effects 

 of atmospheric influences; among these, the sandstones of Stenton, 

 Whitby, Tintern, Rivaulx, and Craigleith, the magnesio-calciferous 

 sandstones of Mansfield, tlie calciferous sandstone of Tisbury, the cry- 

 stalline magnesian limestones, or Dolomites, of Bolsover, Huddlestone, 

 and Roche Abbey, the oolites of Byland, Portland, and Ancaster, the 

 shelly oolites and limestone of Baruack and Ham Hill, and siliciferous 

 limestone of Chilmark, appear to be amongst the most durable. To 

 these, which may be all considered as desirable building materials, we 

 are inclined to add, though they may not always have the e\idence of 

 ancient buildings in their favour, the sandstones of Darley Dale, Hum- 

 ble, Longannet, and Crowbank, the magnesian limestones of Robin 

 Hood's Well, and the oolite of Ketton. 



If, however, we were called upon to select a class of stone fur the 

 more immediate object of o\u- inquiry, we should give the ])reference 

 to the limestones, on account of their more general uniformity of tint, 

 their comparatively homogeneous structure, and the facility and 

 economy of their conversion to building purposes ; and of this class 

 we should prefer those which are most crystalline. 



In conclusion, having weighed to the best of our judgment the evi- 

 dence in favour of the various building stones which have been brought 

 under our consideration, and freely admitting that many sandstones as 

 well as limestones possess very great advantages as building materials, 

 we feel bound to state that for durability, as instanced in Southwell 

 Church, &c., and the results of experiments, as detailed in the accom- 

 panying tables ; for crystalline character, combined with a close ap- 

 proach to the equivalent proportions of carbonate of lime and carbonate 

 of magnesia ; for uniformity in structure ; facility and economy in 

 conversion ; and for advantage of colour, the magnesian limestone, or 

 dolomite, of Bolsover Moor and its neighbourhood, is in our ojiinion 

 the most fit and proper material to be employed in the proposed new 

 Houses of Parliament. 



We iiave the honour to be, my Lord and Gentlemen, 



Your very humble and obedient servants, 

 (Signed) Charlks Barry. 



H. T. Dii Da Becue. 

 Wii.r.iAM Smith. 

 Charles H. Smith. 

 Loudon, March llj, 1839. 



\ye shall give the tables referred to in the rei)ort iu «w »e.\t Joianal, — 

 Spitor. 



ON THE OBSTRUCTION OF STREAMS BY DAMS. 



(From the .American Railroad Journal.) 



Mil Essaij on the Obstrnclion of Sireants by Dams ; with Formula for 

 asurlaining the rise of water caused by their construction. By S. A. 

 Roebling, Cieil Engineer. 



When a stream is to be obstructed by a dam, for the purpose of 

 creating a water-power, making a slack-water navigation, or feeding 

 a canal, it is a matter of importance to know how high the water will 

 rise above its former level in time of freshets. 



Owing to the want of proper investigation, notions contradictory to 

 common sense, have been entertained by professional men on this sub- 

 ject, and the consequence has been, that their works have not idealized 

 their expectations. With a view of throwing some light upon this 

 very important subject, the following illustrations and deductions, 

 based upon the theory of Da Buat and Eyielivein, are offered to the 

 public. 



To compute formula; fur the rise of water by dams, it is necessary 

 to know the amount of water discharged by a freshet, the average 

 width of the stream, its average depth and area of cross section. 



But the gauging of a large stream in high water is a difficult.matter, 

 and at the period when the construction of a dam is to be commenced, 

 there is generally no time to wait for a freshet, for the purjiose of 

 making the desired measurements. I would therefore propose, fur 

 ascertaining the greatest discharge of water, to guage the river when 

 at its medium height. For this pur|iose, let a cross section of the 

 stream be taken, and the velocities of the surface measured at each 

 sounding. It has been ascertained by exjieriments, that the velocity 

 of water, in streams, ckcreasta towards the bottom for every foot 

 depth : 



0.008 

 wdiere v signifies the velocity at the surface. If we now put the depth, 

 for which the average velocity is to be asc-ertained, equal to /;, and 

 denote the required average velocity by v', then we have the velocity 

 at the bottom equal to 



r— 0.008 V h 

 From the surface velocity and bottom velocity we find the average 

 velocity : 



t'-t-ii— 0-008 y. A _,^., , 



V' :^—^ =v — 0'004 V. h 



or, V' z=v (I — 0-004 h) 



When the average velocity, for each sounding, has been thus cal- 

 culated, we can find the discharge per second, in cubic feet. 



For ascertaining the discharge of a river, in time of a high freshet, 

 let its width equal to /. By dividing / into ^le area of the cross sec- 

 tion which has been measured, we get the average depth of the water, 

 which may be represented by //. The area of the profile, divided into 

 the discharge, gives us the average velocity of the whole section, 

 which may be represented by v. The average velocity of a stream in 

 different stages of the water, are, according to Buat and Eytelwein, as 

 the square root of the different average depths. 



Now, let us represent the average velocity of a cross section of a 

 high flood by v' and the average depth of that section by h' ; 



Then is v : v' : : |//i : ■^/h' -. 



therefore, «' = I'^rr = » * / ,- 



'\/h '\/ h 



The average velocity of a high freshet, thus fuund, multiplied into 

 the area of its cross section, gives us the required discharge. 



The above method should be applied, if the necessary measure- 

 ments can be taken, when the stream is at or near its medium height. 

 Without those data, however, an approximate result can be obtained 

 by the formula: 



where v is the average velocity in feet per second, a the area of the 

 profile in superficial feet, h the fall of the river for a certain length 

 I in feet; ;j signifies the perimeter of the profile, not including the line 

 of surface. 



The product of the area into the velocity, thus found, will give the 

 required discharge. This formula, however, caimot be relied on when 

 tlie stream is irregular; it applies with accuracy only to smootli and 

 regular channels and to canals. 



The velocities with which water is discharged through a horizontal 

 opening in the side of a vessel, are according to the laws of gravity, 

 in proportion to the square roots of the respective heights of the 



cyluuiiis of water above the orifices, The pressvu:e, wliich toe partieles 



