766 TRANSACTIONS OF THE AMERICAN INSTITUTE. 



minerals, such as gj^psum, oxide of iron, phosphate of iron, sul- 

 phate of baryta, sulphate of strontia, silicate of copper, carbonate 

 of copper, carbonate of lead, and fluor spar. 



The Atlantic Cable. 



The time required for the transmission of a signal by the Atlan- 

 tic cable, has been found to be thirty-one hundredths of a second, 

 which is probably not in error by one hundredth of a second. At 

 this rate the electric current or force would move under the ocean 

 6,020 miles per second, while on land lines it is estimated to move 

 16,000 miles in the same time. The perfect insulation of the wires 

 of the two Atlantic cables was lately demonstrated by Mr. Lati- 

 mer Clark, who telegraphed from Ireland to America and back 

 again with a battery formed in a lady's thimble. Mr. Collett has 

 sinoe transmitted messages with a batterj^ composed of a gun-cap 

 with a strip of zinc attached by a drop of water, the simple bulk 

 of a lady's tear. 



The cable has lately been brought into important service in de- 

 termining the difference of longitude between points in Europe 

 and America. The telegraphic method of determining longitude 

 originated with S. C. Walker, of the United States Coast Survey. 

 Within the last twenty years the longitude differences of a series 

 of points on the United States coast have been determined with 

 an accuracy far surpassing any work of a similar kind done in 

 Europe. The longitudinal relations of places on opposite sides of 

 the Atlantic have until lately rested on the chronometric calcula- 

 tions made by the United States Coast Survey more thau ten years 

 ago. Fifty chronometers Avere transported between Liverpool, 

 England, and Cambridge, Mass., three times in each direction. 

 The difference in the longitude of these places Avas determined 

 with a probable error of nineteen hundredths of a second, which, 

 although sufficiently precise for commercial purposes, is not so for 

 astronomical purposes. Under the direction of Dr. B. A. Gould, 

 of the Coast Survey, certain points have been determined by 

 means of the Atlantic telegraph, with a probable error of only 

 four hundredths of a second. This will be more clearly under- 

 stood by stating that 1,900 miles have been measured, and the 

 probable variance from the true distance does not exceed forty 

 feet. It is gratifying to knoAV that American savans were the first 

 to make these important determinations. 



