424 



SCIENCE. 



[N. S. Vol. I. No. 16. 



The use or disuse of tMs capital initial 

 may not be a matter of much importance, 

 but if there were no rule upon it there 

 would be lack of that uniformity which is 

 so much to be desired. If left to personal 

 choice, some writers would use it and others 

 would not. The British Association Re- 

 vised Code (1865), the code of the French 

 Zoological Society and that of the Inter- 

 national Zoological Congress leave the 

 matter to individual preference. The code 

 of nomenclature of the American Ornitholo- 

 gists' Union (canon viii.) expresslj' de- 

 cides against capitals, although agreeing 

 ' that it is a trivial matter.' The Inter- 

 national Botanical Congress of 1867 and the 

 committee of the American Association 

 (1894) agree as to its adoption. Therefore, 

 in addition to the above mentioned reasons, 

 botanists write these classes of specific 

 names with an initial capital for the sake 

 of uniformity in botanical writings. 



F. H. Knowlton. 



DENSITY AND DIA3IETEB OF TERRESTRIAL 

 PLANETS. 



Recent determinations of the mass of 

 Mercury have brought out a relation be- 

 tween the densities and diameters of the 

 terrestrial planets which have not hereto- 

 fore been thought possible on account of 

 the supposed great density of Mercurj^. 



The accompanying sketch shows graph- 

 ically this relation. The planets have been 

 plotted with their diameters in miles as 

 abscissa and their density, the earth as one, 

 as ordinates. It is seen that these points 

 lie approximately in a straight line. The 

 data has been taken from Harkness' ' Solar 

 Parallax ' and Young's Astronomy. The 

 masses from the former and the diameters 

 from the latter, except that the density of 

 Mercury is that lately announced by Back- 

 lund from a discussion of Encke's comet. 



The probable error of the density has 

 been obtained by combining the probable 



errors of the mass and diameter, and is 

 shown in the sketch by the arrow-heads 

 above and below the plotted points. It 

 will be seen that the earth, Mars and the 

 moon have much smaller probable errors 

 than Mercurjr and Venus, since these latter 

 have no known satellites to aid in deter- 

 mining their masses. If the most probable 

 straight line be drawn with respect to the 

 former, it will be as shown in the drawing. 

 This line passes mthin the limits of the 

 probable errors of all except Venus. 



It will be observed that the straight line 

 when prolonged to the left does not pass 

 through the origin of coordinates, but cuts 

 the ordinate at some distance above it. 

 This indicates that a planet with a very 

 small diameter would still have a consid- 

 erable density. Meteroic stones of small 

 diameter, when they reach the earth, do 

 have a densitj^ about the same as that of 

 terrestrial rocks, and this is about the den- 

 sity which is indicated in the drawing. 



If this relation should prove to be the 

 true law, then the mass of a terrestrial 

 planet could be determined from its diam- 

 eter. The mass of Venus so determined 

 would be about one-tenth greater than as 

 given. Venus is the only one of the five 

 that is any more discrepant than might be 

 expected from its probable error. The 

 probable error of this planet as given may 

 be too small. An increase of one-tenth in 

 the mass, or a decrease of one-thirtieth in 

 the diameter, would make Venus accordant. 

 A sulficient increase in her mass would 

 explain the movement in Mercurj^'s peri- 

 helion. If the mass of Mercury proves to 

 be as small as now supposed, that is about 

 one-thirtieth that of the earth, it maj' ex- 

 plain some of his irregularities. 



Prof. Young has pointed out that a body 

 200 miles in diameter near the sun would 

 not be likely to be accidentallj' discovered, 

 although it might be seen with some of the 

 best instruments during transit across the 



