ACADEMY OP SCIENCES. 177 



north of Winnemucca, the temperature of which was at least 160 

 degrees. Many diatoms were found with them. 



Professor Davidson was present with an instrument improved by 

 himself, to determine the difference in longitude between two dif- 

 ferent points. He entered, before the Academy, upon a very mi- 

 nute explanation of the modus operandi : 



The principle on which his apparatus works is by instituting an electric and 

 a galvanic current between the points, and on breaking the connection at 

 one point, it registers at the other the precise moment at wliich the break 

 occurred. He illustrated by supposing the two points to be San Diego and 

 San Francisco, (which two points he has lately been working between). The 

 current is established by wire connection above ground, the ends of the wire 

 being sunk in the earth, making the earth supply one-half of the circle. Au 

 operator at each point is supplied with the necessary instruments, which in- 

 clude a clock or barometer, so connected that the instant the break occurs it is 

 noted with the accuracy of a hundredth part of a second. Then making allow- 

 ance for the time of the transmission of the electric wave, and comparing the 

 time at each point, would give the exact difiference in longitude. Extraordi- 

 nary accuracy is attained by the aid of a transit instrument. 



This instrument is a combination of improvements made by himself, by which, 

 with the same instrument, both latitude and longitude can be determined with 

 great facility and accuracy. 



Professor Davidson also explained his rapid method of taking latitude. His 

 combination instrument has been adopted by the corps of Topographical Engi- 

 neers, and the engineers of Europe are taking pattern from it. The two old 

 style s'parate instruments cost $2,100. This combined instrument costs $1,200. 



Dr. Gibbons said he had several times made reference in the 

 Academy to oscillations of his hanging barometer, which he had at- 

 tributed to slight earthquake motion, but some members thought 

 what he saw was but the effect of the wind. He had since noticed 

 several times the same phenomena. On the 9th and 10th of April last 

 he noted several distinctly, and on the 11th, according to the papers, 

 there was a terrible earthquake in China, killing 2,800 people and 

 overthrowing buildings. His theory of earthquakes was that some 

 of them were caused by the gases of the earth passing from one 

 sub-surface cave to another. It was but natural to suppose that 

 just prior to an earthquake the pent-up gases should render the 

 crust of the earth in a state of tension in various points. He had 

 no doubt that the barometrical motion was the result of earthquake 

 movements. 



