Page:Popular Science Monthly Volume 38.djvu/685

Rh per square inch), they are probably stronger than the hand-made; but, as this pressure is uniform for all the briquettes, which is not the case when they are made by hand, the comparative value of the tests is far superior to anything attainable by hand-made briquettes. The following table shows the difference in tensile strength between hand- and machine-made briquettes. Each result is the mean of ten briquettes broken at the end of six months:

All the briquettes used in the tests from which the table and diagrams here given were taken were allowed to stand twenty-four hours in the air, and were then immersed, the time of immersion being the zero marked upon the diagrams, and all the periods of time being reckoned from this point in weeks, which are noted along the bottom of the diagram. A number of briquettes were broken each day for the first seven days; after this a number was broken every seven days, and the average of these results giving the ordinates to the line on the diagram. Besides these briquettes, ten extra ones were broken at the expiration of one week, one month, three months, and six months. The average of the tensile strength of these, and the time of breaking, are shown on the diagram by black dots, the letter showing the brand of the cement: M, Milwaukee; U, Utica; G, Gibbs English Portland; and B, Buckeye American Portland. This system of breaking briquettes shows the effect of time upon their strength. The testing-machine used in these tests was Riehle Brothers' "Standard Cement Tester," in which the strain upon the briquette is gradually increased by means of a screw-and-worm gear. Although the type mentioned by Prof. Griffin possesses accuracy, and is very satisfactory, still the Riehle machine gives equally satisfactory results, and allows of a much greater number of briquettes being broken within a given time.

Any comparison of the relative value of cements based upon their percentage of increase in strength, as made by Prof. Griffin, is of no value. A cement that attains a certain strength in seven days, even if it only increases one per cent during the following ninety days, is superior for constructive purposes to one that increases four hundred per cent during the same time, provided the ultimate strength of the latter is not greater than the former.

The strength of Milwaukee cement, of which Prof. Griffin has much to say, can be seen in the diagram, as compared with the other brands of cements given. The table given by Prof. Griffin,