414 



NA TURE 



[August 29, 1895 



In this form it is easy to see that each term may be graphically 

 represented by an area, and the equation simply expresses the 

 fact that the rectangular area xu is equal to the algebraic sum 



of the areas / udx and / xdu. It is obnous that for txjriodic 



JO J " 



motion the rectangle xu will vanish when a suitable value is 



1*1 j"f 



given to /; but so also will the areas / «</.rand / xdu. So that 



when xti = o we get, either 



I udx = o and / xdu = o ; or / udx = - j xdu. 



-■^ain, in what Clausius calls "stationary motion" when xu 

 docs not vanish pcriixlically, although we can make the ex- 

 pression —ux vanishingly small, by taking / very great, it is 

 2/ 



obvious that if the aieas / udx and - / xdu are not equal be- 



fore multiplying them by — , the expressions so obtained are not 

 so afterwards. Moreover, and finally, it should be observed 

 that the expression /« j udx does not represent iitulic energy ; 



to represent which the expression should be in j udii. The 



•' 

 above considerations seem to me to entirely upset Clausius' 

 demonstration. 



In the tenth edition of Maxwell's " Heat " (p. 323), Lord 

 Rayleigh has given an illustration of the manner in which he 

 sup|»ses the " virial " to act in opposition to kinetic energy, 

 and we may take his illustration as a simple test of the theorem. 

 He suppijses two IxKlies, each of mass /«, to revolve in a circular 

 path with a constant velocity about their centre of gravity. 

 Here, as there is no pressure, the so-called virial equation takes 

 the form 



2j«;r- = i,2\ir. 



In the above equation v, the velocity, is constant, and R = ////". 

 If we take p as the radius of the circle, then r = 2p, and the 

 . equation liecomes 



^TrSm = 4 X 2f)/Sm. 

 Hence 



iv- = p/: 



which equation does no/ refiresent the ordinary law of <enlrijugal 

 fone. Lord Rayleigh omitted to notice that 



2K = :i>ii/=/im = 2m/. 



When, however, we throw overboarti all ideas of " virial," 

 and look upon the term J2R>- in the so-called " virial equation" 

 as simply representing work and equal to j/V, also an expres- 

 sion for work, then the equation 



Sjwz-'' = IpV + KtKr 



is certainly true. But there seems no possible advantage to be 

 obtained in splitting the right-hand member into two equal 

 terms, instead of writing the equation 



2J/«f^ = 3/>V ; or 2J/«zr = 5R/- ; 



in cither of which forms — the first for jireference — it is applicable 

 to ideal gases. For natural permanent gases the equations 

 become, either 



i^Bmz" = 3/V ; or 2j|8//;z'' = IRr, 

 and not 



2i8"iT''' = ZaUr, 

 as given in my letter (p. 221) on " .\rgim and ilii' Kimiic 

 Theor)." C. K. Haskvi. 



I ,,i,l,.,, w \,igust 14. 



Incubation among the Egyptians. 



AUTiFifiM inriitntii.n, like many another practice supposed 



to ' jvilisation, is but a revival from very 



«■' . an author who wrolealKint forty years 



• rii of the Christian era, tells how Ihc 



' 'vith their own hands, bring eggs t(> 



y ri ■ ■ hi' kens thus proiluced arc not 



ii' i liy I he U'iual mcins. 



1 lis differing little from th'ise 



NO. I34S, VOL. 52] 



of ancient times, survives to the present day among the felKahs 

 of Egypt. In suitable places ovens are erected, and the pro- 

 prietors go round the neighbouring villages collecting eggs. A 

 sufficient nuniber having been collected, they are placed on m.ats 

 strewed with bran, in a room about 11 feet square, with a flat 

 roof. Over this chamber, which is about 4 feet high, there is 

 another built about 9 feel in height. The roof, which is vaulted, 

 has a small aperture in the centre to admit liglu dining the 

 warm weather ; below it another opening of larger dimensions 

 conuiiunicates with the oven below. In the cold weather both 

 are kept closed, and a lamp is kept burning within. Entrance 

 is then obtained from the front of the lower chamber. In the 

 upper room fires are made in troughs along the sides, and the 

 eggs are placed on the mats below in two lines, corresponding 

 to and immediately below the fires. The fires are lighted twice 

 a day, the first time to die about midday, the second to last from 

 about 3 p.m. to 8 p.m. The first batch of eggs are left for 

 about half a day in the warmest situation, after which they are 

 moved to make rotmi for others, until the whole number in hand 

 have had the benefit of the position. This is repeated for six 

 days. Each egg is then examined by a strong light. All eggs 

 that at this stage are clear are rejected, but those that are 

 cloudy or opaque are restored to the oven for another four days. 

 Then they are removed to another chamber, where there are no 

 fires, but the air is excluded. Here they lie for five days, after 

 which they are placed separately, about one or two inches apart, 

 and continually turned. This last stage generally takes six or 

 seven days. During this time a constant examination is made 

 by placing e.ich egg to the upper eyeliil, when a warmth greater 

 than that of .the human skin is a favourable sign. The duration 

 of the process generally extends over twenty-one days, but 

 thin-shelled ej^s often take only eighteen <lays. The average 

 heat required is 86° F. Excessive heat is prejudicial. In 

 Eg)'pt the best time is from February 23 to April 24. 



I. TVRKKl.l. Hwi.i-.i. 



Mountain Sickness. 



I HAVE just come back from a journey in the region of the 

 Andes, and in looking overthe numbers of Nati'RK, which had 

 accumulated during my absence, I came across the extract, 

 which you make in your notes of February 21, from the A'et'ue 

 Scienti/it/ne, on the subject of mountain sickness. I cannot 

 agree with M. Kronecker's statement that beyond three thousand 

 metres mountain sickness attacks all jiersons as .soim as they 

 indulge in the least muscular efi'ort, as I made the acquaintance 

 of many people, mostly railway men, living and working at 

 altitudes of fourteen or fifteen thousand feet on tlie Oroya line 

 and the Southern Railway of Peru, who had never experienced 

 soroi/ie, or mountain sickness. As far as my own experience 

 goes, in three journeys across the .\ndes and several n\ountain 

 ascents, including one to the top of the crater of the .Misti, 19,300 

 feet above sea level, I had only one attack of sonv/ie, and 

 that was at the end of a ride on an oil engine from sea 

 level to fourteen thousand feet in nine hours. But this was so 

 complicated with suffocation I >y the oil fumes iind scorching by 

 the heat of the furnace while ruiming through the filty-sevcn 

 tunnels on the line, that I cannot say how nnich was mountain 

 sickness and how much w.as not. .At any rale, I was perfectly 

 well the next morning, and rode over a pass nearly seventeen 

 thousand feet high witliout the slightest inconvenience. .\s 

 regards the danger of a prolonged sojourn, my experience leaches 

 me that it is almost entirely due to jiersonal idiosyncrasy and 

 unwise eating and <lrinking. .\ healthy person whose lungs and 

 heart are all right, who does not over-eal and is very moderate in 

 the use of stimulants, will not suffer from mountain sickness after 

 the first few hours, and in many cases will not sufl'er at all if the 

 nscenl is sulTicienlly gradual. Of course very violent exertion 

 produces distress by reason of the deficiency of oxygen. I do 

 not think that there need be any difficulty about tlie ofiicials 

 of the propose<l Jungfrau railway, if steady men, not of a full 

 habit of body, are selected. I never heard of any Iroulile from 

 mountain sickness among the Peruvian railway men unless they 

 over-stimulated, and yet they are accustomed to go in a day 

 from sea level to 15,764 feet on the Oroya line, and to 14,666 

 feet on the Southern line, and rclurn lo sea level on the follow- 

 ing day. I may add thai I have maile both lliese journeys 

 mysell'^withoul the slightest inconvenience, and ha\e luen able 

 to walk and ride without any trouble at the end of them. 



Lonilon, August 20. CKdRCK <j'kii IITII. 



