be levelled or stepped off, and there should be placed a layer of rich concrete between the rock and the masonry.

If an ice-break with an inclined cutting edge be necessary for any pier that rests on a yielding foundation, a corresponding ice-break or similar offset should be placed at the downstream end of the pier also, even if its appearance be as incongruous as would that of a cowcatcher at the rear of a railway train; for, unless the foundation be thus balanced about the centre of gravity of the vertical load, the portion directly under the superstructure will tend to settle more than that under the nose, and will thus cause a cracking of the masonry and a splitting-off of the front of the pier. Such a disastrous result of the violation of the principle of symmetry in designing is by no means unknown, even in important railroad bridges.

In respect to timber-and-concrete caissons for masonry piers, the following general remarks will prove useful to the designer:

There should be an offset of not less than two feet all around the base of the masonry, and preferably a little more at the ends, so that in case the caisson be located a little out of place the masonry can be shifted thereon so as to bring the pier into proper position. The number of courses of 12" X 12" timber in the roof of the caisson should never be less than four and seldom more than eight. Any less number than four would be liable not to give the roof the proper stiffness during the sinking, and any more than eight would tend to cause an undue settlement of the pier on account of the compression of the timber, which always takes place. The designing of the roof and sides of the working-chamber should be done with the greatest care, so as to prevent all possibility of collapse, and the cutting edge should be shod with steel plates to protect the timber when the caisson is passing through boulders or logs. The roof-timbers, if possible, should always be of full length, and the spacing of the bolts therein should not exceed four feet. The vertical timbers on the outside of the working-chamber  should  be  carried  well  up  into  the  roof, shouldered, and firmly bolted   thereto.   The   crib   above  the  working-chamber  should