October 15, 1886.] 



BCIJEjSCE, 



347 



Baubree has produced artificially one variety (Iherzo- 

 lite) by dry fusion, but this appears to be the first 

 clear case of a peridotite volcano with peridotiteash. 

 Perhaps an analogous r-aseisia Elliot county, Ken- 

 tucky, where Mr. J. S. Diller has recently described 

 an eruptive peridotite which contains the same acces- 

 sory minerals as the peridotite of Kimberley, and 

 also penetrates and encloses fragments of carbonifer- 

 ous shale, thus suggesting interesting possibilities 



H. Carvill Lewis. 



The eccentricity theoiy of the glacial period. 



I desire to add a supplementary note to my letter 

 of Aug. 16, published in the issue of Science for 

 Aug. 27. 



In that letter I called attention to the contrast be- 

 tween the northern and the southern hemisphere m 

 respect of glaciation, as tending to show, that, other 

 things being equal, a climate of means (mild winters 

 and cool summers) is more favorable to the accumu- 

 lation of snow and ice than a climate of extremes 

 (cold winters and hot summers). The bearing of 

 this jjroposition upon the eccentricity theory is pointed 

 out in my letter. 



I now wish to call attention to another well-known 

 geographical fact, which seems to confirm the conclu- 

 sion that glaciation is favored by a climate of means 

 rather than by a climate of extremes. I refer to the 

 altitude of the snow-lme in torrid, temperate, and 

 frigid zones respectively. At the equator the snow- 

 line falls below the annual isothermal plane of 32° 

 F. ; while, as we recede from the equator, the snow- 

 line rises above the plane of 32°. So far does the 

 snow-line rise above the isothermal plane of 32°, as 

 we go polewards, that, while the latter plane reaches 

 the sea-level not far from 60° latitude, it has been 

 doubted whether in the northern hemisphere the 

 snow-line anywhere reaches the level of the sea. 

 According to J. D. Forbes, "'the mean temperature 

 at the snow-line near the equator is 34.7°; in the 

 temperate zone it is 25.3° ; in the arctic regions, about 

 21°" (Johnston, Physical atlas of natural phenomena, 

 Edinburgh and London, 1856, p. 38). While all 

 such numerical statements of the temperature of 

 the snow-line in different latitudes can be con- 

 sidered only rough approximations, there can be no 

 doubt of the general law that (apart from local ab- 

 normaUties) the temperature of the snow-line falls as 

 we go from the equator towards the poles. Now, it 

 is also true that the annual range of temperature in- 

 creases from the equator to the poles. At the snow- 

 line near the equator, the extreme summer tempera- 

 ture is but little above the freezing-point ; while at 

 the snow line in the arctic regions, though the mean 

 temperature for the year falls several degrees below 

 freezing point, the extreme summer temperature 

 rises far above it. The comparison of the zones of 

 climate leads, accordingly, to the same conclusion as 

 the comparison of the northern and southern hemi- 

 spheres. The existence of perpetual snow is shown 

 by both comparisons to depend less upon cold winters 

 than upon cool summers. 



A very simple a priori consideration suggests the 

 probability of the same conclusion which we have 

 drawn from geographical facts. It seems probable, 

 a priori, that extreme winter cold cannot greatly 

 increase the amount of snow-fall. So long as the 

 temperature of any place keeps below 32°, the pre- 

 cipitation will be all in the fornrof snow ; and this 



will be the case when the temperature is but little 

 below 32*^, as truly as when it falls far below zero. 



Cooling a mass of air from 82° to a lower tempera- 

 ture can produce but little additional precipitation, 

 since the maximum vapor tension at 32° is very little, 

 and the change of maximum vapor tension corre- 

 sponding to changes of temperature in the lower part 

 of the thermometric scale is very slight. The influx 

 of warm and moist air bearing supplies of vapor is 

 not favored by extreme winter cold, since such ex- 

 treme cold tends to increase barometric pressure over 

 the area affected. On the other hand, every degree 

 that the summer temperature rises above 32° shows 

 an effective increment of the melting-power of the 

 summer sun. The inference would seem to be justi- 

 fied, that, in any place where the annual mean tem- 

 perature is below or not much above 32°, the more 

 nearly the extreme summer and winter temperatures 

 approach the annual mean, the greater will be the 

 tendency (other things being equal) to the accumu- 

 lation of perpetual snow. This a priori inference 

 seems to be in exact accord with the geographical 

 facts referred to in this and in my former letter. 



William North Eice. 

 Wesleyan university, Oct. 8. 



The theory of utility. 



In connection with the suggestive article in Science 

 of Oct. 1, on ' Launhardt's Mathematical economics,' 

 I would like to offer a new theory of utility, or, rather, 

 to discuss it from a new standpoint, and indicate 

 what I consider to be the eri'or in Jevons's main 

 premise. 



Utihty, or usefulness, is the satisfying of desires. 

 Desires are always in the present, though many, per- 

 haps the most of them, have a prospective nature. 

 Usefulness is not the capacity or capability of being 

 useful : it is the state or quality of being useful. It 

 involves, not a possible, but an actual satisfying of 

 desires : e.g., on a certain day a loaf of bread would 

 have possessed utility for Eobinson Crusoe in satisfy- 

 ing his hunger ; a second loaf would have possessed 

 utility, not in satisfying the hunger of the morrow, 

 but in satisfying his desire to have the possible wants 

 of the future provided for. 



If utility be defined as a capacity to serve man or 

 to satisfy his desires, and by this is meant something 

 quite different from the actual satisfying, it serves 

 no purpose of distinction, for with this definition, 

 when affirming utility to bean attribute of any thing, 

 we must always add, ' under certain circumstances ;' 

 and there is probably not a thing in existence but 

 what, under certain circumstances, possesses this 

 capacity. 



The confusion prevailing as to the nature of utility 

 has arisen from the fact, that, in discussions upon the 

 subject, the provident trait in man's character has 

 been entirely neglected ; for from this trait spring 

 desires which are, indeed, of a prospective nature, 

 but whose satisfaction involves utility as indubitably 

 as does the satisfaction of his physical needs. 



Utility being of the present moment, time is not 

 one of its dimensions, as the theory of ' final degree 

 of utility ' necessarily presupposes. When Jevons 

 (' Theory of political economy,' p. 51) declares that 

 "utility may be treated as a quantity of two dimen- 

 sions, — one dimension consisting iu the quantity, and 

 another in the intensity of the effect produced upon 

 the consumer," — it is clear that the supposed di- 

 mension of quantity does not have reference to the 



