period of time and comprising enough individual samples of the cement to establish a rigid standard for that particular brand. It should include neat tests and also those mixed with every proportion of sand that is liable to be used on the work, to ascertain as well what mixture of sand will produce the requisite strength. During construction work cement is liable to be delivered in such quantities that it is not possible to make long-time tests without working a hardship on the contractor. If, however, a standard has been established, and it is definitely known that a certain strength neat in seven days will develop into a certain other strength in thirty or ninety days mixed with the specified amount of sand, a very accurate and satisfactory conclusion can be arrived at. Each cement, however, must have its own standard, and the operator who makes the original tests should be retained to carry them on during the prosecution of the work.

No new cement should be accepted on short-time tests. They are often very deceptive. Unless it has been used and gained a reputation, careful and elaborate tests should be made as detailed above. The briquettes should be mixed neat and with the proportions of sand determined upon, the same day and by the same person, using the same sample of cement for both neat and sand briquettes, so that the loss of strength occasioned by the added sand can be accurately determined. Long-time tests are absolutely necessary, as a few cements with a moderate amount of sand will give practically as great a strength as when tested neat. As it is longtime results that are desired in construction, the importance of this can be readily seen. Table No. 21 clearly illustrates this.

The above is the average of five briquettes, and the cement is a natural product well known in the New York market. Thirty per cent of water was used in the neat mixture and 14 per cent in the sand.

Mr. E. B. Noyes, in Journal of Engineering Societies for June 1896, gives a case in point when a good cement was rejected and a poorer one accepted on comparatively short-time tests without apparently any previous knowledge. Table No. 22 gives his

The cement was an American brand, and the briquettes were mixed one part cement to one part of sand. Nos. 1 and 2 were not used on account of their poor showing in their first tests, while at the end of the year their superiority was clearly demonstrated. No. 2 was certainly a remarkable specimen, and any engineer would be justified in rejecting it upon the six months' test without having had any previous knowledge of its wonderful recuperative powers. In many works, too, it could not be used notwithstanding its great strength in one year, as its development during the first six months is very slow. Sample No. 4 actually receded in strength, though so little that it might have been caused by some individual briquette. It would seem to be a fair inference that it had practically reached its limit in six months.

The author several years ago had some tests made of the principal American cements tributary to the city where he was then located, practically on the lines as indicated above. The results were very satisfactory, demonstrating the necessity of such action, and in this particular case bearing out some action that had been taken in rejecting certain cements. Table No. 23 gives the results attained.

TABLE No. 23.

CEMENT MIXED NEAT.

Briquettes 2 hours in air, remainder in water.

24 Hours. 7 Days. 15 Days. 30 Days. 90 Days. 6 Months. 9 Months. 1 Year.

CEMENT 1 PART, SAND 2 PARTS, REMAINDER IN WATER.

This shows that No. 1, which was the weakest at the end of a year neat, was the strongest when mixed as it is generally used: and that Nos. 3 and 5, which were two of the highest neat, were but one-half the average strength of the other at the end of the year when mixed with sand.

Some engineers in making cement specifications go very elaborately into the component parts of the material, exacting a certain percentage of one substance and ruling out more than a certain amount of another. This practice is dangerous, unless one is perfectly sure of his standing, or the limits are so elastic as to be of no value. It is really encroaching on the prerogative of the manufacturer. The engineer wishes results, and it is the maker's business to produce a cement that will give them. The manufacturer will have no difficulty in meeting any requirements, but at what cost to the long-time test he alone might be able to tell. Then the products of different mills differ so that a slight excess of one ingredient might be neutralized by that of another. It is well known that many excellent brands of cement are made. It is better to obtain a perfect knowledge of the peculiarities of each.

and, after specifying certain of these, make sure that each delivery is kept up to the standard.

In the case of an excessive demand when the output is small, manufacturers are liable to put on the market a product that in the rush has not received sufficient attention, and which ordinarily would not be sent forth-or it may happen without their knowledge. It is the object of the tests to detect this or similar defects in standard brands.

In the paper by Mr. Lesley previously referred to, he gives the requirements for tensile strength as found in different specifications and shown in Table No. 24.

Cement specifications generally specify a time within certain limits for the initial and final sets. When this is done, and in fact the time of setting is generally noted in all tests, it is necessary to define what is meant by these terms. A standard was first adopted by General Totten at his work at Fort Adams, R. I., previous to 1830. This was that when the mortar would sustain a wire

of 1/12 inch diameter weighted to 1/4 pound, it should be said to have received its initial set, and its final when it would sustain a wire of 1/2 inch diameter bearing a one-pound weight. The actual setting-point must be obtained by frequent trials. This standard. was accepted by Gillmore and others, and is the one in general use at the present time.

While many more and elaborate tests can be and are made on cements, those for fineness and tensile strength on the lines herein indicated will give good and safe results for general work.

For general specifications, then, it would seem to be in accordance with best practice to make the following requirements for fineness:

It is not necessary, however, that all cements should reach these figures. But it should be provided that a quick-setting cement should increase a certain per cent over its 24-hour strength in 30 days, and that a slow-setting cement should not be less than a specified minimum at that time, and particular attention should be given to its strength with the sand mixtures.

Just what requirements should be called for in special cases de