212 THE POPULAR SCIENCE MONTHLY. 



planning of machinery, so that it shall be at once economical and du- 

 rable in operation, and simple and cheap in construction, is not merely 

 an important incidental duty, it is absolutely the chief and most diffi- 

 cult duty of the mechanical engineer. 



■+•+- 



PREPARATIONS FOR THE COMING TRANSIT OF 



VENUS. 



THE nature of a transit of one of the inferior planets (Mercury or 

 Venus) is well understood, and the phenomena attending such 

 a transit have been thoroughly discussed, and fully described in many 

 places. The importance of the observation of these transits, and the 

 general character of the results expected from the expeditions sent 

 out to observe them, are probably understood by all, but it is thought 

 that a brief account of the means that are to be employed to accom- 

 plish the desired end will be of interest. 



The records of the plans which have been formed, and of the prep- 

 arations which have been made by the different governments of the 

 world and by private individuals, are, unfortunately for the general 

 public, published only in proceedings of scientific societies, or in many 

 cases they exist only in manuscript. When the expeditions return 

 home after the observations are made, in astronomical Europe and 

 America will resound the busy hum of preparation, and from the 

 beginning of 1875 the reader of astronomical items will be sated. 



At first will come a series of preliminary reports as the parties 

 come in ; then we shall have the final reports, giving numbers, data, 

 descriptions of instruments, and the observations made at the transit, 

 the longitudes and latitudes of the various stations, and, in short, every 

 result which the practical astronomer will have derived. 



These final reports will be eagerly looked forward to, for upon 

 them depends the constant of solar parallax, and from them will be 

 deduced the definitive result of all the astronomical work done on the 

 globe on that day. 



We know already that the final outcome of all these vast prepara- 

 tions which we are going to describe will be a number very near to 

 8".848. 



The whole world is united in an effort to know exactly how to 

 change this ; whether to write it greater or less. But the results of 

 these expeditions, if they are successful (and we can hardly fail of suc- 

 cess), will be, not simply the establishing of the earth's distance from 

 the sun on a certain basis, but much more. 



So many expeditions of trained scientific observers will bring back 

 with them data only second in importance to the main object of their 



