Fig. 82.

This is readily seen, for the needles straighten out when relieved of the load. The jack-screws are now lowered or the wedges under the uprights eased up; the uprights taken away, needles removed, and the holes filled up. Underpinning operations must be slowly and carefully performed, as they are very risky. If there is any danger of a wall tipping during the operation, grooves are cut into the wall and "shores" or braces placed against it. The feet of the shores rest on cross-planks, same as uprights, and are wedged up to get a secure bearing of the top of the shore against the wall. Where the outside of a wall cannot be got at, "spring needles" are used from the inside. That is, the one end of the needle acts as a lever and supports the wall, while the other, inner end, is chained and anchored down to prevent its tipping up.

The strength of a wall depends, of course, largely on the material used. A good, hard-burned brick, well laid in cement-mortar, makes a very strong wall. To tell a good brick, first examine the color; if it is very light, an orange-red, the brick is apt to be soft. If the brick is easily carved with a knife, it is soft. If it can be crushed to powder easily, it is soft. If two bricks are struck together sharply, and the sound is dull, the bricks are poor; if the sound is clear, ringing, metallic, the bricks are good and hard. If a brick shows a neat fracture, it is a good sign; a ragged fracture is generally a poor sign. The fracture also shows the evenness of the burning and fineness of the material. A brick that chips and cannot be cut easily is a good brick. The darker the brick, the harder burned. This, of course, does not hold good for artificially-colored bricks. The straighter and more regular the brick, the softer it is (as a rule), as hard-burning is apt to warp a brick.

What has been said of the strength of bricks holds good of terracotta. The latter should be designed to be of same thickness, if pos-sible, in all parts, and any hollows caused thereby must be filled-in solid. It is best to fill-in the hollows with bricks and mortar several days in advance, and let the filling set, so as to be sure it will not swell up afterwards and burst the terra-cotta.

To judge of the strength and durability of stones is a more difficult matter. If the stone be fractured, and presents, under a magnifying glass, a bright, clear, sharp surface, it is not likely to crumble from decay; if the surface is dull-appearing and looks earthy, it is likely to decay. Of course, samples can be tested for their crushing and tensile strengths, etc. And we can tell somewhat of the weathering qualities by observing similar stones in old buildings: much, however, depends whether the stones come from the same part of the quarry. Another test is to weigh different samples, when dry; immerse them in water for a given period, say, twenty-four hours, then weigh them again, and the sample absorbing the least amount of water (in proportion to its original weight) is, of course, the best stone.

Another test is to soak the stone in water for two or three days and put it out to freeze; if it does not chip or crack, it will probably weather well. Chemical tests are made sometimes, such as using sulphuric acid, to detect the presence of lime and magnesia; or, soaking the stones in a concentrated boiling solution of sulphate of soda; the stones are then exposed to the air, when the solution crystalizes in the pores and chips off particles of the stone, acting similarly to frost. The stones are weighed before and after the tests, the one showing the least proportional loss of weight being, of course, the better.

If stones are laid on their natural beds, however, little need be feared of the result, if the stone seems at all serviceable. The main dangers to walls arc from wet and frost. Very heavy and oft-repeated vibrations may sometimes shake the mortar-joints, but this need not be seriously feared, in most cases; machinery may often cause sufficient vibrations to be unpleasant, or even to endanger wood or iron work, but hardly well-built masonry. Of course, the higher a building is, the greater will be the amount of vibrations and their strength. For this reason it is advisable to place the heaviest machinery on the , lowest (ground) floor. The beds of such machinery should be as far as possible from any foundations of walls, columns, etc., and the beds should be independent and isolated from all other masonry. Malo, in Le Genie Civil, recommends the use of asphalt for machinery-foundations, as they take up the vibrations and noise, and are as solid as masonry, if properly built. His claim seems well founded, and has been demonstrated practically;

Strength of stones.

Machinery foundations.

the asphalt foundation not only preventing vibrations but stopping the sound. A wooden form is made, covered inside with well-greased paper; into this are placed slightly-conical shaped wooden bars and boxes, also covered with well-greased paper, which are secured in the places to be occupied by the bolts and bolt-heads, and arranged for easy withdrawal. A layer of melted asphalt a few inches thick is then poured into the mould; over this are dumped heated, perfectly clean, sharp, broken stones and pebbles, rammed solid, the pebbles filling all interstices; then more asphalt is poured in, then another layer of stones and pebbles, etc. It is claimed that this foundation becomes so solid that it will not yield enough to disarrange the smooth running of any machinery, while its slightly-elastic mortar, besides avoiding vibrations and noise, prolongs very much the durability and usefulness of the machinery. Two dangers must be guarded against; viz., the direct contact of oil or heat with the asphalt. Stationary drip-pans guard against the former, while a layer of rubber, wood, cement, or other non-conductive material would accomplish the latter object. Where noise from machinery is to be avoided, a layer about one inch thick, of hard rubber or soft wood, should be placed immediately under the engine-plate. If this layer were bedded in asphalt the precaution would be still more effective.