THE RELATIVE VALUE OF CEMENTS. 667 



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 re- 

 sult 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 im- 

 mersion 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 bri- 

 quettes 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 expira- 

 tion of one week, one month, three months, and six months. The 

 average of the tensile strength of these, and the time of break- 

 ing, 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 pos- 

 sesses 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 in- 

 creases 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, 



