878 Transactions of the American Institute. 



arm fixed upon the apparatus, allows of recording this eigiial upon 

 a landmark in the field of the instrument. Finally, a hairline, pro- 

 duced by means of a thread situated at the hight of the optical 

 axis, shows all the points in the photograpliic panorama, in which 

 the higlit is equal to that of the station whence it operates. The 

 employment of the apparatus requires no other special knowledge 

 than that of photography. To comprehend the gi-eat simplification 

 thus brought about in the art of surveying, let us repeat the details 

 of the methods hitlierto used : 



After having, with considerable trouble directed the optical axis 

 of the telescope in the theodolite separately upon each signal, the 

 figures of the levels are taken down in succession. When the levels 

 are very numerous, several hours are necessary for making one 

 entire tonr of the horizon, A protractor is now placed in position, 

 and, after having been noted, is directed in succession to horizontal 

 angle. These several operations are very long and tedious, and 

 often errors are committed which are more difficult to correct the 

 higher they mount up. The ordinary theodolite is only really pre- 

 ferable when it is desired to obtain the most precise results by calcu- 

 lating the construction of each triangle. 



On the otlier hand, the photographic plane-table will complete 

 tour of the horizon within half an hour, and the number of signals 

 may be multiplied w^ithout in any wa}* rendering the task more 

 laborious. By increasing the ^lumber of original signals there is less 

 risk of error, and of subsequent rectification, and much intermediate 

 work and calculation ol)viated. Less cars and skill are required in 

 its employment, and inclemency of weather becomes a matter of 

 little I'mportance. The committee state that the instrument has 

 already been used with great advantage, and in some cases errors 

 have been discovered in standard maps by its use. They say, in con- 

 clusion, that after twelve years of incessant study, Mr. Chevalier lias 

 died at the moment when he had hoped to see his invention adopted 

 by learned bodies, and suggests that so novel and ingenious an appli- 

 cation should be honored by the high approbation of the Academy 

 of Sciences. 



Electkicity. 



The view long entertained by some Amei'ican scientists regarding 

 electricity is similar to that expressed by the Rev. Father Secchi of 

 Rome, in a letter addressed to M. F. Mazco at Turin, the following 

 extract from which appears in the Paris Les Mo:ide : " I believe that 



