Page:Popular Science Monthly Volume 4.djvu/225

Rh 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 Washington, 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 agreement of the results obtained over three different cables, by different observers at different times. This agreement is so marvelous (considering the independence of the determinations), that the results are here quoted:

It must be remembered also that, incidentally as it were, the relative 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 determinations 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 Venus's disk is tangent to the limb of the sun. To make this observation, a small equatorial telescope 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 distance 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