698

THEORY AND PRACTICE OF ENGINEERING.

Book II

To drill the holes in very hard stone, it is necessary to have a steel cold chisel about 2 feetlong, and the breadth of the hole required, the edge being double bevelled, and not toosharp; a workman holds this instrument where the hole is to be made, whilst anotherstrikes it, taking care that the drill is turned partly round, or kept revolving during thesuccession of blows; as the indentations are made, the powdered stone or dust is removedfrom the hole. When rocks are blasted by gunpowder, the holes are drilled to a greaterdepth, usually 16 or 18 inches deep, and an inch or more in diameter; the powder isintroduced into them, sewed up in a linen bag, with a cartridge ‘made of tin; dry sand isput upon it and rammed down, and the top of the hole is filled with moistened sand. Atrain of powder in a fine tin tube is connected with the holes, to which is attached a slowmatch or some wild-fire, so arranged as, when fired, to give the workmen time to retire outof all danger from the scattering of the fragments, which occasionally is attended withgreat violence. Voltaic electricity, conducted by means of wires, has been effectuallyapplied to the ignition of the gunpowder, which is attended with infinitely less casualties,inasmuch as if ignition does not immediately take place, it is not to be feared afterwards.

In the selection of stone for the purposes of construction, regard must not only be hadto the colour and texture, but also to its power of withstanding the exposure to atmo-spheric agency, or the decomposing effects of water. Stones are composed of various earthysubstances in such a state of hardness as not to be softened by immersion in water underordinary circumstances; they may be classed under three distinct divisions, viz. the sand-stones, limestones, and granites.

Sandstones are formed of angular or rounded grains of different earths or minerals, whichare either held together by a cement or base, or joined without any such basis by simplejuxta-position: when the grains composing them are small, they are called sandstone, butwhen they increase in dimension, they are designated conglomerate, if the particles arerounded; if angular, they take the name of brescia. The consolidation of the varioussandstones seems to have taken place not during the time they -were forming under water,but when they were upheaved, or had the water drawn from them; this is implied from thefact that most sandstones, when first taken from the quarry, are softer than after exposure tothe air ; no doubt the water which they contain is speedily evaporated, and the minerals itholds in solution become deposited, and by their crystallisation give greater hardness andconsistency to the mass, binding it more firmly together, and rendering it more difficult tocut; some varieties are so plastic, when first taken from the quarry, that they may be easilycompressed, and afterwards, from the loss of the water they contain, become perfectly hard.Sandstones are divided into three varieties, according as the quartz or siliceous grains com-posing them are cemented by siliceous, argillaceous, or calcareous matter.

In the siliceous kinds the particles are cemented by a base of quartz: in the argillaceousby a base of clay, usually impregnated with a red oxide of iron, which gives that tint to thewhole rock ; and in the calcareous kinds by a marly or calcareous cement, and it must beobvious that the quality of sandstone entirely depends upon the durability of its cementingproperties. As the particles which are held together, being silex in nearly a pure state, arenot acted upon by either air or water, its decomposition is found to commence by the de-struction of its base, which liberates the grains in the sandstone, or permits them to crumbleinto fine sand : when the base is highly impregnated with silica, it is very hard, has almostthe character of porphyry, and seems to defy decomposition.

In the varieties of sandstone with small grains, set in an argillaceous or calcareous base,the colours vary from red, grey, green, yellow, and brown, and are arranged in zones orbands dependent upon the oxidation of the iron in the base or cement. This variety ofsandstone is found alternating with beds of red-coloured clay, or marl, which is sometimesslaty, or mixed with sand and mica, passing into sandstone slate; the beds are of greatthickness.

From sandstone being formed in strata, and laminated, it is obvious that when applied tobuilding purposes, they should be so placed as to correspond w-ith their natural beds, for ifin any other position, or vertical to their planes of stratification, any weight placed uponthem would tend to cleave or split them into laminae.

The sandstones that have undergone an analysis consist of: —

Cragleith.

Darley Dale.

Heddon.

Kenton.

Mansfield.

Silica ...

98*3

98-40

95-1

93-1

49-4

Carbonate of lime

1-1

0-36

0-8

2-0

26-5

- magnesia

0 0

0-0

0*0

o-o

16-1

Iron alumina

0-6

1-30

2-3

4-4

3-2

Water and loss -

o-o

1-94

1-8

0'5

4-8

Bitumen

o-o

o-o

00

o-o

o-o

Specific gravity of dry

2-338

masses

2-232

2-628

2-229

2-247