76 ROUGH WAYS MADE SMOOTH. 



any modification of the former mean estimate of the earth's 

 mass on account of the unexpectedly large value deduced 

 from the Harton experiment. 



It appears to me probable that a similar fortune will 

 attend the latest measurement of the sun's distance. But 

 fortunately the matter will not rest merely on measurements 

 already made. Many fresh measurements will be made 

 during the next few years by methods already tried and not 

 (like Delisle's transit method) found wanting. The recent 

 close approach of the planet Mars was not allowed to pass 

 without a series of observations specially directed to the 

 determination of the sun's distance ; and we know that 

 observations of Mars are among the most advantageous 

 means available for the solution of this difficult problem. It 

 was indeed from such observations that the first really trust- 

 worthy measures of the sun's distance were obtained two 

 centuries ago. The small planets which travel in hundreds 

 between the paths of Mars and Jupiter have also been 

 pressed into the service. And now so many of these are 

 known that scarcely a month passes without one or other of 

 them being favourably placed for the purpose of distance 

 measurements. For this too their star-like discs make these 

 bodies specially suitable. 



The most probable inference respecting the results ob- 

 tained by the British expedition is that their chief value 

 resides in the evidence which they afford respecting the 

 Delislean method of observation. They seem to demonstrate 

 what had before been only surmised (though with consider- 

 able confidence by some astronomers), that this method can- 

 not be relied upon to correct our estimate of the sun's 

 distance. In the transit of 1882, which by the way will be 

 visible in this country, we may be certain that other and 

 more satisfactory methods of observation will be employed. 



Before concluding, it may be well to make a few remarks 

 upon some misapprehensions which seem to. exist as to the 

 propriety in the first place, and the desirability in the second, 

 of comments upon the arrangements adopted by Government 



