114 Q. H. Williams — Petrographical Microscope. 



perience has forced upon me, that wherever there is one seismo- 

 scojoe there must he two. That is to say, when any locality 

 is selected as a suitable place for a seismic station two complete 

 stations must be established, separated sufficiently so as not to 

 be liable to the same accidental disturbances but near enough 

 to render it reasonably certain that a real seismic disturbance 

 which affects one will also affect the other. If separated by a 

 distance of from a few hundred feet to a quarter or half a 

 mile, the necessary conditions would be satisfied. Two such 

 stations ought to sensibly agree as to the exact time of any dis- 

 turbance and not only would they enable the observers to dis- 

 tinguish accidental or false alarms, but they would afford excel- 

 lent checks upon each other on the occasion of a genuine re- 

 cord. 



It is hardly necessary to refer to the interesting and impor- 

 tant results which would almost certainly come from the or- 

 ganization of one hundred or even fifty such stations wisely 

 distributed, at first over those sections of the country known to 

 be most subjected to earthquakes. On the hypothesis of trans- 

 mission through a homogeneous, elastic medium, records at less 

 than a half dozen stations will suffice to determine the velocity 

 of transmission, and the coordinates of the origin of the disturb- 

 ance. Although in the case of the earth this hypothesis is not 

 tenable, it is an approximation to the truth, and there can be 

 no doubt that the mean of a number of such determinations, 

 based upon different groups of observations would have consid- 

 erable weight in the discussion of the dynamics of the problem. 



I think it will be generally admitted that the management 

 and direction of an investigation so extensive as the territory 

 involved could only be successfully carried out by the govern- 

 ment ; and that the Director of the Geological Survey, of whose 

 disposition in the matter there can be no doubt, should be fur- 

 nished by Congress with the authority and material for its 

 accomplishment. 



Art. IX. — On a new Petrographical Microscope of Ameri- 

 can Manufacture ; by George H. Williams. 



The importance of the microscope in geological investiga- 

 tions — particularly in the domain of the crystalline rocks — is 

 now universally recognized, even by those geologists who do 

 not themselves employ it. The light already shed upon some 

 of the darkest and most intricate problems by recent petro- 

 graphical methods, uncertain though it sometimes be, is full of 

 promise for the future. Geology is reaping almost as great 



