93 PLEASANT WA YS IN SCIENCE. 



difficulties which had for a long time foiled Huggins, and 

 which Secchi was never able to master, rendered the first 

 Greenwich measures of stellar motions in the line of sight 

 wildly inconsistent, not only with Huggins's results, but with 

 each other. 



Secchi was not slow to note this. He renewed his objec- 

 tions to the new method of observation, pointing and illus- 

 trating them by referring to the discrepancies among the 

 Greenwich results. But recently a fresh series of results has 

 been published, showing that the observers at Greenwich 

 have succeeded in mastering some at least among the dif- 

 ficulties which they had before experienced. The measure- 

 ments of star-motions showed now a satisfactory agreement 

 with Huggins's results, and their range of divergence among 

 themselves was greatly reduced. The chief interest of the 

 new results, however, lay in the observations made upon 

 bodies known to be in motion in the line of sight at rates 

 already measured. These observations, though not wanted 

 as tests of the accuracy of the principle, were very necessary 

 as tests of the qualities of the instruments used in applying 

 it. It is here and thus that Secchi's objections alone re- 

 quired to be met, and here and thus they have been 

 thoroughly disposed of. Let us consider what means exist 

 within the solar system for thus testing the new method. 



The earth travels along in her orbit at the rate of about 

 1 8^ miles in every second of time. Not to enter into nice- 

 ties which could only properly be dealt with mathematically, 

 it may be said that with this full velocity she is at times 

 approaching the remoter planets of the system, and at times 

 receding from them ; so that here at once is a range of dif- 

 ference amounting to about 37 miles per second, and fairly 

 within the power of the new method of observation. For 

 it matters nothing, so far as the new method is concerned, 

 whether the earth is approaching another orb by her motion, 

 or that orb approaching by its own motion. Again, the 

 plant Venus travels at the rate of about 21^ miles per 

 second, but as the earth travels only 3 miles a second less 



