0)i the Bai'oinetrlc Measurement of HelgJUs. 81 



The true difference of level is 237 feet,* and the distance in 

 a strait line twenty-foui" miles, the intervening country being 

 rangy and timbered. The worst result by instrument No. 1, 

 is 12 feet in error, the best 1 foot, and the mean 1"3 feet ; 

 by No. 2, the worst is 24;'2 in error, the best So, and the 

 mean ^G ; by No. 3, the worst is 191 in error, the best '3, 

 and the mean 7'^. The mean of the three means is 44 in 

 error. 



I also read the instruments at the various stations 

 between Melbourne and Lilydale, and plotted the section of 

 the Inie from the observations so made. The diagram 

 submitted shows the result. I do not regard this as a f;xir 

 test, as the readings were hurried, and the instruments had 

 not time to settle. Still the result, especially with No. 3, 

 the Watkin, is fairly consistent and good enough to be of 

 very considerable practical utility. 



1 think the above experiments will serve as far as they 

 go, to show that barometer work, carried out with fairly 

 good instruments carefully used, and with all proper 

 corrections applied, may be relied upon to give differences 

 of level with greater accuracy than is often supposed, and 

 with sufficient accuracy to be of very great service in 

 preliminary investigati(jns connected with engineering 

 works of various descriptions. 



Art. X. — A Neiu System of Photo-Lithography. 

 By G. W. Pekry, Esq. 



[Read June 18, 18&y.] 



I have since been informed that this level is not quite accurate. 



G 



