the cost of such work of repairs, and the amount of the same shall be paid to the contractor on the certificate of the engineer that the work has been completed to his satisfaction; and, after such certificate shall have been issued, the contractor shall again become responsible for all damage that may be caused by the action of the water, in the same manner as is specified above.

If such appraisal of the engineer is not satisfactory to the contractor, the said contractor shall so state in writing to the aqueduct commissioners, and, thereupon, a board of arbitration, composed, first, of the chief engineer, or of such other person that the aqueduct commissioners may designate; second, of a person selected by the contractor; third, of another person to be designated by the other two, shall proceed to appraise the cost of such damage, and their decision shall be final and binding on both parties, provided it is the unanimous decision of the three members of the said board; but if the said decision is not unanimous, the appraisal of the chief engineer shall stand and become final and binding to both parties. And, on the cirtificate of the aqueduct commissioners that the said appraisal has been made in accordance with the stipulations of this agreement, the amount of said appraisal shall be paid to the contractor. And the said appraisal, whether made by the chief engineer or by the said board of arbitration, shall include only the cost of the actual work done to repair the damage, and shall not include any alleged loss of profit or other loss due to the delay caused by such repairs, but an extension of time shall be granted to the contractor for the performance of his contract, equivalent, in the opinion of the engineer, to the loss of time due to the interruption of the operations of construction on account of the said work of repairs.

The contractor is to do all the draining and pumping which shall be necessary for keeping the work free from water, and if at any time the engineer is of the opinion that, in order to maintain the slopes and sides of the excavations in proper order, it is necessary to remove the water from the ground outside of the limits of the excavations, the contractor shall, at his request, sink the necessary pipes or wells to intercept the water, and place, maintain and work such pumping or other exhausting apparatus as shall be sufficient to properly maintain the said slopes and sides.

The cost of furnishing the necessary appliances and machinery, of working them, and of doing all the work connected with draining and pumping operations, is to be included in the prices bid for the various kinds of work which the draining and pumping operations are intended to protect.

A. F.

This is a new departure in arbitration proceedings, but it has many things to recommend it.-AUTHOR.

SPECIFICATIONS FOR CEMENT MORTAR, CONCRETE, AND MASONRY.

127. Cement Mortar. There are in general two kinds of cement in common use in America, namly, Portland or artificial cement, and Natural cement. Portland cement is an artificial mixture of lime and clay properly burned and ground. Natural cement is made by burning the natural rock which contains approximately the proper ingredients, and grinding the calcined product. Portland cements are known by their various manufacturer's names or brands, and are mostly imported from Germany, France, and England. Recently a number of manufactories have been established in America. Natural cements are usually known under a geographical name, indicating their place of manufacture, as Rosendale cement, made on the Hudson river; Louisville cement, made on the Ohio river in the vicinity of Louisville; Utica cement, made at Utica in the northern part of the state of Illinois; Milwaukee cement, etc. In general the Portland cement costs about three times as much as the natural cements and it has three or four times the strength of these. It is common to require a tensile strength of from 300 to 400 pounds per square inch for Portland cements, which have hardened one day in the air and six days in water, and about 100 pounds per square inch tensile strength for natural cements, similarly treated. The Louisville cement is quick setting, and a very fair test may be obtained of its strength in twenty-four hours, in which case a tensile strength of from 60 to So pounds per square inch may be specified, the briquettes being allowed to remain one hour in the air, and twenty-three hours in water.

*The reader is referred to a small book of 100 pages by F. P. Spalding assistant professor of civil engineering, Cornell University, Ithaca, New York, on "The Testing and use of Hydraulic Cement" (1893). This is a most excellent hand-book for the engineer as well as for the student; it fully describes the characteristics of both natural and Portland cements, as well as the most approved methods of testing cements and of mixing mortars.

The strength of cement and cement mortar depends greatly on the fineness of the cement. This is usually tested by passing it through a sieve of from 50 to 100 meshes per lineal inch, having from 2,500 to 10,000 meshes per square inch. The 100 mesh sieve is much to be preferred, and is usually specified in the case of Portland cement, since probably only the particles which would pass through such a sieve are really efficient or active in the process of hardening, the coarser parts being inert, or as so much sand.

A cement mortar is a thorough mixture of sand with cement, first in a dry state, usually in the proportion of one of cement to two of sand by measure, with natural cements, and one of cement to three or four of sand when Portland cement is used. After these ingredients have been effectually mixed, sufficient water is added to reduce the composition to the desired consistency. It is important that the sand should be clean, or free from all earthy ingredients. It is common also to specify that it shall be sharp; that is to say, the grains should not be too much rounded. Ocean beach sand is apt to be very much worn, and not sharp in this sense. River or bank sand is

usually preferred on this account.

In specifying the proportions of sand and cement to be used in making up a cement mortar, it is customary simply to name so many parts of sand to one part of cement, by measure. It would, as a rule, be inconvenient to determine this ratio by weight, but a determination by measure is subject to serious objections. For instance, a barrel or original package of cement, when dumped or turned out upon a mixing platform in a loose and fluffy condition will have nearly 50 per cent. more volume than it had in the original package. It is necessary, therefore, in order that the meaning of the specifications shall be clear, to indicate whether the proportions by volume shall be taken with the cement in the original package, or in a loose state, after having been emptied from such package. It is per

haps more convenient to measure the cement after it has been emptied from the original package. In any case the engineer should decide which method he proposes to adopt, and reveal this decision in the specifications themselves. The following specification for the making of cement mortar is satisfactory in every respect, except that it does not indicate whether the cement is to be measured in the original package, or in a loose condition. Since all kinds of cement set or harden by a kind of crystallizing process, and in some cases this action begins almost immediately after the cement has been wet down, it is necessary to use the mortar as quickly as possible after it has been mixed. In the case of Portland cement, and some of the slow setting natural cements, an hour or two may be allowed to elapse between the wetting of the mortar and its final use upon the work, but, in the case of such quick setting cements as the Louisville, the mortar should be finally placed in the work within thirty minutes of the time it is first wet. If cement mortar is used after it has begun to set, it will be permanently weakened, and if used very long after it has begun to set, it will remain inert and will never 'harden.

Mortar shall be composed of one measure of cement and two measures of sand, and shall be mixed on a tight platform as follows: One measure of sand shall be evenly distributed on the platform; and one measure of cement shall be distributed on the sand, and a second measure of sand shall be distributed on the cement. The sand and cement shall then be thoroughly mixed in a dry state, being turned over with shovels until this is accomplished. Water shall then be added in a sufficient quantity to convert the sand and cement into a mortar which will stand in a pile and not be fluid enough to flow. During the application of the water the mass must be constantly turned with shovels, so that the mortar will be of uniform consistency. O. B.

128. Cement Concrete. Cement concrete is usually composed of cement mortar as described in the previous article, mixed with broken stone. It may, however, be composed of cement mortar mixed with gravel. If gravel can be procured free from earthy matter, varying in size from coarse sand to

stones not more than about two inches in diameter, it would serve a better purpose in the manufacture of concrete than does broken stone. Experiments have shown also, that when stone is broken in a stone crusher and not screened, so that all the finer parts remain in, including the stone dust, a stronger concrete results than with the use of the same quantity of screened

stone.

The ideal cement concrete is such a mixture of material of graded size, from the largest used down to the finest sand, as will make a nearly solid mass, when properly mixed. This may then be solidified by uniting with it such an amount of finely ground cement as will serve to completely coat each and every particle of sand, gravel, or stone, and fill the small voids remaining after the graded materials have been thoroughly and uniformly mixed. Since crushed rock is always angular it will be often impossible to make as solid a concrete mass with it as can be made by the use of gravel. When gravel is used it is best to have it screened to a series of regularly graded sizes, and then such proportions of each successive smaller size used as will serve to fill the voids in the larger size. The cement finally fills the voids between the small sand grains.

The sand and cement should always be very thoroughly mixed dry, then the coarser material should be thoroughly wet and the excess of water drained off, after which the mixed sand and cement should be incorporated with the moistened gravel or rock, and a sufficient amount of water added while the mixing is in progress as will reduce the entire mass to the proper consistency. The most effectual mixing can be done by machinery, but it is more commonly done by hand. Perhaps the best cement mixer is a cubical box mounted on trunions at its diagonally opposite corners into which the proper proportions of the constituent parts, including the water, are placed and the whole given a certain number of revolutions. There are various kinds of continuous mixtures into which the proper