528 ANNUAL RECOHD OF SCIENCE AND INDUSTRY. 



of stone, jointed with such nicety that even the blade of a 

 penknife could not be pushed between them, but without 

 any mortar whatever. On the other hand, the Egytians used 

 mortar freely, as in the Great Pyramid. This, however, con- 

 sisted almost wholly of gypsum, or sulphate of lime. A spec- 

 imen of mortar from an ancient Phoenician temple was like 

 that found in the baronial castles of England, having been 

 made of burned lime, fine sand, coarse sand, and gravel. The 

 lime in it had become completely carbonated, converting 

 the mass into solid rock. The ancient Greek mortars, from 

 ruins in the vicinity of Athens, contained more lime than that 

 from Cyprus, and no gravel. That from Herculaneum,Rome, 

 and its neighborhood, appeared to have been made from 

 burned lime and volcanic ash, or puzzuolana. The mortar 

 of the Great Pyramid consisted principally of hydrated sul- 

 phate of lime, to which were added traces of carbonate of 

 lime, carbonate of magnesia, alumina, silica, and a very small 

 per cent, of water. 3 A, Ajyril 26, 1873, 05. 



ACOUSTICS OF LARGE EOOMS. 



A recent study of this subject has been made by Orth, 

 according to the method of graphic construction as detailed 

 in the treatise of Langhaus, the law of reflection of sound, 

 and that of intensity, as affected by varying distance, being 

 mainly involved. The former consideration is easily intro- 

 duced in graphic constructions; the latter necessitates calcu- 

 lations based upon the length of the j^ath of the sound waves. 

 By the employment of 0.01 of a square meter, at a distance 

 often meters from the origin of sound, as a unit of intensity, 

 Orth was also able to accomplish a graphic representation 

 of intensity. The effect of intersecting waves upon each oth- 

 er he did not regard as demanding practical consideration ; 

 but, on the other hand, the diffusion of sound by reflection 

 from rough surfaces proved to be of great importance, as 

 furnishing, in some cases, the readiest remedy for acoustic an- 

 noyances. The only source of acoustical defects in a hall to 

 be considered, according to these investigations, is the reflec- 

 tion of sound waves in such a way that they strike upon the 

 ear of the hearer within a certain interval after the direct 

 waives, and are recognized as an echo or resonance. For 

 graphic determinations this interval of tune is converted into 



