50 BULLETIN OP THE 



received at an elevation of only 2 feet. The greatest difference 

 was observed in continued slow rain, when the air was saturated 

 with moisture ; the least difference in sudden showers of short 

 duration. 



Remarks were made by Mr. Henry aud Mr. Hilgard, the 

 former referring to observations made in one of the towers of the 

 Smithsonian Institution. 



Mr. E. B. Elliott read a paper on 



THE MUTUAL RELATIONS AS TO PRICE OP GOLD, SILVER BULLION, 

 SILVER COIN, AND GREENBACKS. 



Mr. 0. T. Mason read a paper on 



THE classifications OP OBJECTS OP ARCHEOLOGY. 



Mr. Gill called attention to the distinction between archae- 

 ology and ethnology, and suggested some changes in Mr. 

 Mason's classification. 



Mr. J. B. Hilgard gave an account of 



THE MEASUREMENT OP A BASE-LINE POR THE PRIMARY TRIANGU- 

 LATION OP THE UNITED STATES COAST SURVEY NEAR ATLANTA^ 

 GEORGIA. 



Three separate measurements of this line were made ; the first 

 two at low temperatures, the measurements being made in oppo- 

 site directions, in order to test the effect of slopes upon the per- 

 formance of the apparatus The third was made in summer at a 

 high temperature, in order to test the accuracy of the correction 

 for temperature. The essential features of the apparatus were 

 described, and the mode in which compensation of the two 

 measuring bars for temperature is effected — the system being to 

 compare these compound measuring bars with a simple standard 

 bar of iron of six metres in length at the time of measuring the 

 base, and as nearly as practicable at the average temperature 

 which obtained during the measurement. The mechanical com- 

 pensation of the bars is thus only required to maintain a nearly 

 uniform length during the variations of temperature in the 

 measurement, and the actual length at the mean temperature of 

 measurement is taken account of by the known dilatation of the 



