These figures only give the approximate weight, and are useful as showing at a glance the weight that is put on the walls by each truss. In order to get a more exact computation, the number of cubic feet of timber in each truss must be calculated and multiplied by the weight of the timber per cubic foot, and to the amount so obtained must be added the total weight of all iron straps, bolts, or other fastenings that are used. Should the trusses carry a ceiling, the weight of the portion that is carried by each truss has to be added before the total amount carried by the walls and the trusses themselves can be correctly estimated. In calculating the weight that is borne by any piece of construction, its own weight is an important factor, especially when a heavy material like iron or steel is adopted.

Since preparing the bulk of this chapter I have been enabled, by the courtesy of the architects, to whom I am greatly indebted, to present to the reader a fine modern example of constructional carpentry in the oak footbridge at Guildford.

In this age of cement and iron it is pleasant to find an example of a bridge built in honest English oak. The new footbridge over the Wey at Guildford, which was formally opened by the mayor and corporation on August 25, 1909, is a picturesque structure, as well as a substantial one. The bridge, which cost about £400, was paid for by public subscription, and the subscribers and the inhabitants generally are to be congratulated upon having secured a structure which is appropriate to its purpose and its situation, and is in every way a desirable addition to their beautiful and interesting town.

The design of this bridge, which was chosen from among more than seventy submitted in competition, was modelled on the design of Palladio's bridge over the river Cismone, between Italy and Germany.

The clear span of the bridge over the river Wey and towing-path is 63 ft. 6 in., and over all the bridge is 78 ft. in length, and is 5 ft. wide in the clear.

At the west end the bridge is approached by a staircase, so as to leave a headway of 9 ft. to the towing-path, and at the east end the bridge lands the pedestrian on the Shalford-road level.

The bridge is constructed entirely of English oak, framed together and secured with wrought-iron straps and bolts.

The dimensions of the principal timbers are: Tie beams (each in three lengths), principal braces, queen posts, and straining-beam, 12 in. by 9 in., and the cross beams carrying the floor, 9 in. by 9 in.

The bridge is supported at each end on four 12-in. by 12-in. oak posts braced together, on sole-pieces resting on cement concrete foundations. The detailed drawing, reproduced on the folding plate, will make the methods of construction clear to practical readers.

The architects are Messrs Clements & Moon, of Guildford, and the contractors, Messrs R. Wood & Son, of Guildford. Messrs Henry Adams and Son, of 60 Queen Victoria Street, E.C., acted as consulting engineers.

CHAPTER XVIII.

THE CONSTRUCTION AND MAINTENANCE OF ROADS.

Road Materials. The making and repair of roads are very important parts of a municipal engineer's business; in fact, county engineers devote most of their time to overseeing and looking after the upkeep of the roads under their charge.

During the latter part of the nineteenth century the roads, which together with the canals had been the sole means of inland communication, fell of necessity into much disuse, and consequent neglect. During the last ten years, however, the advent of the motor car has changed all this, and now the question of road maintenance (not so much construction) is a very vexed one, especially as most of the roads in outlying country districts are totally (or were so at least before the profession took up the matter) inadequate to deal with the fast and heavy traffic of motor cars. Nothing is more detrimental to the reputation of a man in a public position than bad roads.

Road "metal," as the stone used in roadmaking is called, must be viewed from several standpoints; and when the surveyor is called upon to discriminate between different classes of road stone, he must carefully regard the following considerations, viz.

1. Cost

2. Durability f

these are very important.

3. Cleanliness, that is freedom from dirt, clay loam, soil, etc.

4. Weathering properties.

5. Uniformity of wear. A road which will wear unevenly is very undesirable, as it becomes pitted and difficult to repair.

6. Ease of traction.

7. Ease of foothold for horses.

Therefore, from the foregoing conditions we conclude that the essential properties of a good road stone are

1. Hardness.

2. Durability.

3. Toughness.

4. Binding properties.

5. Maintenance of a rough surface.

This latter condition is only essential in sett paving stones which would wear to a smooth and slippery surface; such as hard grauite would be unsuitable. The softer granites are always used for this purpose.

Now, before going on to describe the composition of the various kinds of road stone, it is essential that the reader be first well acquainted with the several minerals which go to make up a road stone. First of all we have

1. Biotite. It is a mineral of the mica group, but is black in colour and contains some iron and manganese. It is soft, has a perfect cleavage, and will break up into thin, smooth scales.

2. Apatite is a natural phosphate of crystalline rocks. It gives off much phosphoric acid, and it is upon the acid that all vegetable life is dependent. It occurs principally as phosphate of lime, and is found in hexagonal prisms. It is soluble in dilute nitric acid. Its colour is principally sea-green, and is found in small, transparent, long-pointed crystals, which are invisible to the naked eye. It also goes under the name of "asparagus" stone.

3. Hornblende is a black or dark-green lustrous mineral. It will decompose with the formation of chloride, and is very like augite in appearance, but can easily be distinguished by its natural angle of cleavage of 124 degrees.

4. Orthoclase is potash felspar.

It is sometimes combined with soda, and occurs in white, pink, or grey crystals. Decomposition is liable to set in, yielding kaolin and quartz.

5. Kaolin is china clay, and is found to a great extent in Cornwall. It is a hydrated silicate of alumina and a pure white powder. It is principally composed of granite in a state of decomposition. By the use of strong H2SO, we can make alum from it.

6. Olivine is a ferrous and magnesian orthosilicate, and occurs in rhombic prisms of an olive-green colour. It will readily weather to serpentine.

7. Muscovite is potash mica.

another name for mica.

It is transparent and flexible, and only

8. Pyroxene is composed of silica, iron, and oxide of alumina. Its colour will vary; it does in fact range from white to dark green, or even black. crystals may be rhombic or monoclinic.

The

9. Pyrites is disulphide of iron. It occurs usually in cubes of a brassylooking colour. It is very brittle. Burnt in air, it will yield sulphur dioxide gas. When it occurs in an isometric form it gets the name of marcasite. 10. Plagioclase is triclinic felspar. Usually occurs in clear transparent crystals. It will decompose frequently into china clay (= kaolin).

11. Quartz is a very important mineral in the composition of road stones. Being crystallised silica, it is the principal ingredient of sandstone. It is a very hard mineral, being even harder than steel, but is very brittle. Its appearance is smooth and glassy. In its colourless form it occurs as rock crystal. What is known as quartzite is a white, grey, or yellowish rock, chiefly composed of quartz.

12. Felspar is a silicate of alumina. It may contain potash, soda, or lime, and usually of a white or grey colour. It will decompose upon exposure to the atmosphere, and lose its glassy appearance thereby. It occurs in two main groups, monoclinic and triclinic.

13. Felsite is a fine-grained igneous rock of the acid variety.

14. Augite occurs in all igneous rocks, especially basalt. It is usually in black monoclinic crystals of short and stout shape. It has the property of decomposing into "greenstones" such as serpentine.

The Properties of Road Stones in general.-The stones used by surveyors for road purposes are usually natural inorganic rocks. As stated, all stones must be hard, tough, and durable. Many stones which on examination would have a naturally good appearance in the quarry, would, when exposed to the air, become rotten. This would be due to various chemical actions, set up in a great measure by horse manure. It may here be stated that the fact of

being able to judge with fair accuracy a good road stone by sight is a valuable acquisition of the road surveyor. It is, of course, only gained by practical experience. To attempt the question here would be ridiculous; all we can say is, the practice should be cultivated.

One of the best rocks for use in macadam roads is granite. It is what is called a plutonic igneous rock, and was formed under great heat and pressure. It is holocrystalline and an acid rock. [What is here termed an acid rock is a rock which is known to contain more than 66% of silica, and the more silica a stone contains the smaller will be its specific gravity.] The average composition of granite is

36% of felspar

11% of mica
46% of quartz

The granite from County Donegal has 65% of felspar. Hornblende is a valuable property of granite. It is, however, liable to yield to atmospheric influences and become clayey. Magnetite and pyrites are detrimental to its quality because they are oxidisers. When granite is used for road metal it must have a fine grain, and it is the possession of this latter property in a marked degree which has made the Guernsey granite so much favoured, especially in the south of England. The Killiney granite, so much valued in Ireland in the Dublin district, is what is called a foliated granite. Most granite will absorb about 1% of its weight in water. When it occurs in bosses, which it usually does, the stone at the top will probably be rotten and unfit for use. A special form of granite is found in Cornwall. Is has special qualities, one of which is, it decomposes into kaolin. It is supposed to have been formed under considerably less pressure than ordinary granite. It is only found in small isolated masses, and is called elvans. It will prove a good road stone. Before we proceed further it will be necessary to explain four common terms in reference to road stones. They are, porphyrite, porphyry, decomposi tion, and alteration.

When a rock has more soda lime than potash lime it is called a porphyrite. When the conditions are reversed it is called a porphyry.

When a rock breaks down, due to chemical action, "decomposition" has set in.

When the structure is altered mechanically, "alteration" is what has taken place. Such alteration often tends to make a rock more compact.

Hornblende sometimes contains felspar. When so, the felspar is usually crystalline; but when quartz is present, it is simply mechanically interlocked; but this interlocking may reach such a pitch that the quartz is actually intergrown. An example of this is what is called graphic granite.

Orientation of a rock is its liability to split (e.g. slates and York stone for paving purposes). The term Basalt is used as a name for most igneous rocks having a very dark colour; they are usually low in silica, and are called Basic rocks; this is in contradistinction to acid rocks containing more silica, they are naturally heavy; metallic oxides are also present in a marked degree; they make very good setts because they do not wear smooth. As road metal (as broken stone is called) they are also useful. The structure is usually compact. For making setts a specially adaptable stone comes from Penmaenmawr in North Wales; they are much used in Manchester, Liverpool, and Dublin, where they appear to have given satisfactory results. The Penmaenmawr stone is itself a cross between a diorite and basic diabase, principally