304 



SCIENCE. 



[Vol. XVIII. No. 460 



within one-fifth of the whole volume. This residue being 

 drained off, the frozen part is allowed to melt under cover, 

 so as to exclude the atmospheric moisture. Chloroform thus 

 refined has, by way of testing its durability, remained ex- 

 posed on the roof in a light brown bottle from November till 

 June without the slightest sign of decomposition. 



Professor Pictet has already taken steps to introduce his 

 process into manufacture, and proposes to apply the princi- 

 ple to various other chemical and technical objects. Sul- 

 phurous ether, for instance, has by a similar process been 

 produced in a hitherto unknown degree of purity. At the 

 same time, the professor continues eagerly to pursue tlie 

 various purely scientific inquiries with which he started. 



R. Du Bois-Reymond. 



LETTERS TO THE EDITOR. 



#** Correspondents are requested to be as brief as possible. The writer^s name 

 is in all cases required as proof of good faith. 



On request in advance, one hundred copies of the number containing his 

 communication loill be furnished free to any correspondent. 



The editor willbe glad to publish any queries consonant with the character 

 of the journal. 



Dr. Hann and the Fohn. 



In the good old meteorological times — before the advent of 

 Dr. Hann and his fatal misapplication of the mechanical theory 

 of heat to the phenomena of the atmosphere — it was generally 

 considered that the hot winds of Switzerland, the so-called Fohn, 

 had their birthplace in the Desert of Sahara. The good folks 

 thought, in their simplicity, that the warm air which suddenly 

 came upon them must have come from a hot place. They noticed 

 that it always came from a more or less southerly direction, and 

 judged that the sand-dust it carried with it must have come from 

 some sandy region. They felt the sand-dust smart in their eyes, 

 and saw how it discolored the snow white face of the mountains ; 

 but the sand-dust did not obstruct their vision further but they 

 could judge that the dust must have got into the air from some- 

 where, and so they 6xed upon the Sahara as the nearest and most 

 likely locality. 



It seems, indeed, marvellous that Dr. Hann' s opposing theory 

 could have made headway against such glaring and incoatestible 

 facts, as it should be much easier to convince the world of a sim- 

 ple and tangible truth than to convert it to the opposite theory, 

 which has nothing whatever of observations of natural phenom- 

 ena to support it; but when I here propose to maintain the old 

 theory, by taking away the basis of Dr. Hann's, it may not seem 

 unbecoming in me to say a few words in apology for such seem- 

 ingly reckless behavior. 



It is now nearly two years ago I published, through these col- 

 umns, the rudiments of a theory of the atmosphere which is more 

 or less diametrically opposite to the prevailing ideas on the sub- 

 ject. Rain was supposed to be due to expansion of moist air, and 

 I found by experiments that it must be due to compression, etc. 

 As far as I could ascertain, meteorologists had no objection to 

 urge against my theory, but on further investigation it became 

 clear to me that they possessed a high-priest, or Dalai Lama, in 

 Dr. Hann, without whose sanction no new theory could be seri- 

 ously considered by reputable meteorologists of any standing, and 

 as he refrained from expressing himself directly on the subject, 

 the matter was put aside for the time being. Dr. Hann, however, 

 gave indirectly vent to his opinion ; a few weeks after the publi- 

 cation of my theory Dr. Hann handed in to the Vienna Academy 

 of Science a paper wherein he held forth that the established the- 

 ories on the atmosphere required considerable modifications, and 

 the modifications he proposed were all an approach towards the 

 views which had immediately before been set forth by the present 

 writer. This paper caused considerable discussion, but nobody 

 seemed to consider the high-level observations on which he pro- 

 posed to base these modifications of any real value. Any further 

 approach to my views would undoubtedly have led Dr. Hann to 

 upset his own theory of the Fohn, — a theory which has brought 

 its author no inconsiderable renown during the past years, — and 

 that any man should upset his own reputation as a philosopher 



could hardly be expected ; and there is so far nothing to be said 

 against his silence, as all is fair in war and love, and to gain time 

 is the great object in all cases of emergency. 



There is, however, a time for every thing, and as nearly two 

 years have elapsed since I published what, in my humble opinion, 

 is the true theory of the atmosphere, it may be about time for 

 me, and my duty also, to endeavor to upset the chief obstacle 

 against its adoption, which I consider Dr. Uann's Fohn theory 

 to be. 



I take occasion from an article by Mr. Rotch, on " Mountain 

 Meteorology,'' in American MeteorologicalJournal (August, 1891), 

 wherein this staunch upholder of Dr. Hann's views has very ably 

 tried to systematize the aspects of the prevailing meteorological 

 theories from this particular point of view. It is always a lauda- 

 ble endeavor, of any author, to try systematically to combine into 

 a collected whole the varying theories concerning any particular 

 branch of this science, as it enables the critic to mark out the 

 weak point. The most consistent or systematic treatise on the 

 atmosphere as a whole, which the present writer is acquainted 

 with, is the " Elementary Meteorology," by Mr. R. H. Scott, and 

 the remarkable candidness of its author made it a comparatively 

 easy task for the present writer to point out, that, according to 

 the causes of rain given there, we should not get any rain at all 

 if we were to believe the gentlemen who had the atmosphere " in 

 charge." so to speak. As I on that occasion dealt extensively with 

 the question of the effect on the humidity of the air caused by 

 ascent or descent of the air, I may at present confine myself to 

 discuss exclusively the question of change in temperature caused 

 by ascent or .descent. 



Mr. Rotch says, on page 154: " It has been shown by Dr. Hann 

 that the Fohn owes its extreme warmth, as well as its dryness, 

 to the descent from the ridges on the north side of the Alps, and 

 that it does not bring it from further south. The warmth of the 

 Fohn is explained by the fact that a mass of air sinking into one 

 of higher pressure is warmed at a rate of one degree for each 300 

 feet of descent, and a rapidly sinking stream of air, which is so 

 quickly heated, must be relatively very dry." And a few lines 

 above we read: "The cool night wind (from the mountains) is 

 caused by the sinking of the cold air into the bottom lands, and 

 is most intense in narrow valleys, where there is great dififer- 

 ence between the temperature of the valley and the plain," and 

 again, during Fohn the temperature " rises sometimes 60° F. above 

 the normal." 



The unbiased reader cannot help noticing the anomaly that 

 the mere descent of air is (1) in case of Fohn accused of causing a 

 considerable rise in the temperature above the normal, and (2) in 

 case of the night wind an equally considerable lowering of the 

 temperature below the normal. 



This contrast brfcomes even more drastic when Mr. Rotch says, 

 page 151, " Slowly descending currents of cold air fill the valleys like 

 rivers, while the summits receive the air warmed dynamically by 

 descending from a greater height; and it seems obvious that the 

 author has a little private theory of his own that it makes a great 

 difference whether the air is descending slowly or not, whereby he 

 tries to patch over the glaring discrepancy. 



We may now set to work to put these contradictory theories to 

 their proper test, thereby confining ourselves to statements con- 

 tained in the article itself. It is thus truly mentioned that the 

 air is warmed at the rate of 1° F. for each 300 feet of descent and 

 cooled at the rate of I" F. for each 800 feet of ascent, but we 

 also find mentioned another fact, equally true, that, on an average, 

 or under normal conditions, the temperature of the atmosphere 

 decreases at the rate of 1° F. for each 300 feet rise; and this fact 

 throws a peculiarly instructive light on the whole subject. It shows 

 that under normal conditions air rising to any height will during 

 the ascent be cooled by expansion at such a rate that wherever it 

 goes it meets with air having a temperature exactly equal to its 

 own, and also that air descending to any level will for a similar 

 reason meet with air having a temperature exactly equal to its 

 own, wherever it goes. In other words, for air arriving at any 

 particular place to have an abnormally high or low temperature 

 it must have had an equally abnormally high or low temperature 

 at the locality where it started from. 



