THE COMING TRANSIT OF VENUS. 213 



journeys. The latitude and longitude of many of the almost unknown 

 islands of both oceans will be established with a certainty as great as 

 the corresponding coordinates of most seaports on our own Atlantic 

 coast. Observations for magnetic constants will be made at places 

 widely separated, and much will be learned in this way. The line of 

 Russian stations, and the American station in Siberia, will be connected 

 by telegraphic wires to St. Petersburg, and possibly the stations in 

 the Indian Ocean may likewise be joined with New York or Wash- 

 ington, so that independent longitude determinations by telegraph 

 may be extended over seven-eighths of the globe. 



Americans should not forget that our own Coast Survey has made 

 three independent determinations of transatlantic longitude in the 

 years 1867, 1870, and 1873, nor should they forget the wonderful agree- 

 ment of the results obtained over three different cables, by different 

 observers at different times. This agreement is so marvelous (con- 

 sidering the independence of the determinations), that the results are 

 here quoted : 



Longitude of Harvard College Observatory, west of Greenwich Observatory. 



Campaign of 1867 4 h 44 m 31 8 00 



Campaign of 1870 4 44 31.05 



Campaign of 1873 4 44 30.99 



Mean 4 44 31.01 



It must be remembered also that, incidentally as it were, the rela- 

 tive longitude of Paris and Greenwich Observatories was found : so 

 that it is to American astronomers, working by a method of American 

 invention, that the exact value of so important a coordinate is due. 



Americans will have reason to be proud if equally exact determi- 

 nations can be extended by them from the Indian Ocean to "New York, 

 and from Siberia to Greenwich. 



These are only some of the incidental advantages which it may be 

 hoped will be gained by the various expeditions for which the different 

 governments have provided. 



There are various ways in which the observation of the transit of 

 Venus may be made, and, in order to describe the instruments, and 

 the preparations which are making, it will be necessary to refer to 

 these briefly : 



1. There is the method of contacts, which consists in determining 

 the time at which the limb or edge of Yenus's disk is tangent to the 

 limb of the sun. To make this observation, a small equatorial tele- 

 scope is needed, provided with suitable colored glasses to protect the 

 observer's eye, and with the usual appurtenances. 



2. The micrometric method, which consists in measuring the dis- 

 tance apart of the bright horns of that part of the edge of the sun 

 which Venus partly obscures as she is moving on or off. As Venus 

 has a sensible diameter (about one minute of arc), it will take a sen- 



