of water is necessary and sufficient to hydrate the cement; less than that amount will be detrimental, while an excess can do no harm, provided, as before mentioned, that it does not wash the mortar from the stone.  Again, dry concrete is apt to be very porous, which in certain positions is a very grave objection to it—­this, not only from the fact of its porosity, but from the liability to disintegration from water freezing in the crevices.

Concrete, when ready to be placed in position, should be of the consistency of a pulpy mass which will settle into place by its own weight, every crevice being naturally filled.  Pounding dry concrete is apt to break adjacent work, which will never again set properly.  There should be no other object in pounding concrete than to assist it to settle into the place it is intended to fill.  This is one of the evils concomitant with imperfection of mixing.  The greater perfection of mixing attained, the nearer we get to the ideal monolith.  The less handling concrete has after being mixed, the better.  Immediately after the mass is mixed setting commences; therefore the sooner it is in position, the more perfect will be the hardened mass; and, on the other hand, the more it is handled, the more is the process interrupted and in like degree is the finished mass deteriorated.  A low drop will be found the best method of placing a batch in position.  Too much of a drop scatters the material and undoes the work of thorough mixing.  Let the mass drop and then let it alone.  If of proper temper, it will find its own place with very little trimming.  Care should be taken to wet adjacent porous material, or the wooden form into which concrete is being placed; otherwise the water may be extracted from the concrete, to its detriment.

It has been found on removing boxing that the portion adjacent to the wood was frequently friable and of poor quality, owing to the fact just stated.  It is usual to face or plaster concrete work after removing the boxing.  On breakwater work, where the writer was engaged, the wall was faced with cement and flint grit, and this was found to form a particularly hard and lasting protection to the face of the work.

Batches of concrete should be placed in position as if they were stones in block masonry, as the union of one day’s work with a previous is not by any means so perfect as where one batch is placed in contact with another which has not yet set.  A slope cannot be added to with the same degree of perfection that one horizontal layer can be placed on another; consequently, where work must necessarily be interrupted, it should be stepped, and not sloped off.