324: 



THE POPULAR SCIENCE MONTHLY.— SUPPLEMENT. 



illustrating the principle of the method. We are 

 not to suppose that on a point so simple the 

 Greenwich observers have been in any sort of 

 doubt. 



At first their results were not very satisfac- 

 tory. The difficulties which had for a long time 

 foiled Huggins, and which Secehi has never been 

 able to master, rendered the first Greenwich 

 measures of stellar motions in the line of sight 

 wildly inconsistent, not only with Huggins's re- 

 sults, but with each other. Secehi was not slow 

 to note this, and a short time ago he renewed 

 his objections to the new method of observation, 

 pointing and illustrating his objections by refer- 

 ring 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 difficulties which they had 

 before experienced. The measurements of star- 

 motions showed now a satisfactory agreement 

 with Huggins's results, and their range of diver- 

 gence 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 al- 

 ready measured. These observations, though 

 rot wanted as tests of the accuracy of the prin- 

 ciple, were very necessary as tests of the accu- 

 racy of the instruments used in applying it. It 

 is here and thus that Secchi's objections alone 

 required 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 eighteen and one-third miles in every 

 second of time. Not to enter into niceties 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 difference amounting to 

 about thirty-seven miles per second, and fairly 

 within the power of the new method of observa- 

 tion. For it matters nothing, so far as the new 

 method is concerned, whether the earth is ap- 

 proaching another orb by her motion, or that orb 

 approaching by its own motion. Again, the planet 

 Venus travels at the rate of about twenty-one and 

 a half miles per second, but as the earth travels 

 only three miles a second less swiftly, and the 

 same way round, only a small portion of Venus's 

 motion ever appears as a motion of approach 

 toward or recession from the earth. Still, Venus 



is sometimes approaching and sometimes reced- 

 ing from the earth, at a rate of more than eight 

 miles per second. Her light is much brighter 

 than that of Jupiter or Saturn, and accordingly 

 this smaller rate of motion would be probably 

 more easily recognized than the greater rate at 

 which the giant planets are sometimes approach- 

 ing and at other times receding from the earth. 

 At least the Greenwich observers seem to have 

 confined their attention to Venus, so far as mo- 

 tions of planets in the line of sight are concerned. 

 The moon, as a body which keeps always at 

 nearly the same distance from us, would, of 

 course, be the last in the world to be selected 

 to give positive evidence in favor of the new 

 method ; but she serves to afford a useful test 

 of the accuracy of the instruments employed. 

 If, when these were applied to her, they gave 

 evidence of motions of recession or approach at 

 the rate of several miles per second, when we 

 know as a matter of fact that the moon's distance 

 never 1 varies by more than 30,000 miles during 

 the lunar month, and her rate of approach or re- 

 cession thus averaging about one-fiftieth part of 

 a mile per second, discredit would be thrown on 

 the new method— not, indeed, as regards its prin- 

 ciple, which no competent reasoner can for a 

 moment question, but as regards the possibility 

 of practically applying it with our present instru- 

 mental means. 



Observations have been made at Greenwich, 

 both on Venus' and on the moon, by the new 

 method, with results entirely satisfactory. The 

 method shows that Venus is receding when she 

 is known to be receding, and that she is ap- 

 proaching when she is known to be approaching ; 

 and the method shows no signs of approach or 

 recession in the moon's case, and is thus in sat- 

 isfactory agreement with the known facts. Of 

 course, these results are open to the objection 

 that the observers have known beforehand what 

 to expect, and that expectation often deceives 

 the mind, especially in cases where the thing to 

 be observed is not at all easy to recognize. It 

 will presently be seen that the new method has 

 been more satisfactorily tested, in this respect, 

 in other ways. It may be paitly due to the effect 

 of expectation that in the case of Venus the mo- 

 tions of approach and recession, tested by the 

 new method, have always been somewhat too 

 great. A part of the excess may be due to the 

 use of the measure of the sun's distance, and 



1 It varies more in some months than in others, as the 

 , moon's orbit changes in shape under the various perturb- 

 ing influences to which she is subject. 



