NOVKMBKH 2, IStCl 



SCIENCE. 



591 



In such a condition as this, the desert- whirls 

 may begin. It is clcarlj' not necessary, in 

 order to produce this result, that the vertical 

 decrease of temperature shoulrl lie as mucli as 

 twentj' degrees in three hundred feet, as in the 

 case just assumed. In order to pass from 

 the stable equilibrium, through the indifferent 

 to the unstable equilibrium, it is sutficient, in dry 

 air, that the vertical decrease should bo greater 

 than !.(>' in three hundred feet, or greater than 

 one degree in one^undred and eight3"-three feet. 

 Moreover, it is important to notice, that, ac- 

 cording to tills theoretical explanation, tlie 

 condition of indifferent equilibrium is passed 

 before the surface-air is, as Franklin (17.53) 

 and Belt (1859) have said, specifically lighter 

 than that above it. This would require a tem- 

 perature difference of at least 5.G° F. in three 

 hundred feet. It is sufficient that the surface- 

 air shall be potentiall}- lighter, though ab- 

 solutelj- (before any motion takes place) 

 heavier, than the higher layers, as Reye first 

 showed (1864) ; or, in other words, stable 

 equilibrium is lost, and indifferent equilibrium 

 reached, when the surface-air is just enough 

 warmer than anj" layer above it to make up for 

 the change of temperatures produced in equal- 

 izing their densities. Anj" further excess of 

 surface-warmth brings about theoretic unstable 

 equilibrium. On the other hand, whirlwinds 

 of decided activity will not be formed until the 

 difference of temperature is much in excess of 

 the narrow limits just given, the strength of the 

 up-current increasing with its excess of warmth. 

 Motion of the atmosphere caused by small dif- 

 ferences of temperature would be very gentle, 

 and would be perceived onh- in the ' boiling ' 

 of the air, often seen in summer-time over the 

 brow of a hill. 



It must be, then, the sun's heat, as was sup- 

 posed, that destro3's the normal stable equilibri- 

 um of our atmosphere ; and to a disturbance of 

 this kind we can refer more or less directly all 

 storms, and, indeed, all winds that blow about 

 the earth. Without the heat that is constantlj- 

 showered down on us, we should soon gravi- 

 tate into :v lifeless condition of stable equilib- 

 rium, cliemical, organic, and physical, and there 

 remain in endless death. But the sun allows 

 no such inactivit}" on its attendant planets : it 

 keeps them alive and at work. 



(To bt continued.) 



THE FRENCH ECLIPSE EXPEDITION. 



P. .1. Janssen, tlie leader of the French e.\pedition 

 which visited Caroline Island to observe the solar 

 eclipse of May 0, has made a report to the French 



academy of sciences, which is published in full in the 

 liuUelin hebdomntlnire tie I' Association scientifique, 

 no. ISl. It contains, first, an interesting account 

 of the voyage to Caroline Island, and a brief descrip- 

 tion of the island, with the difficulties encountered in 

 landing the instruments; then follows a statement of 

 the instrumental outfit and the plan of observations. 

 The search for intra-mercurial planets was assigned 

 to Messrs. Palisa and Trouvelot. The former us^ed an 

 equatorial of 0. l(i m. aperture, havin; a short focus 

 and large field: the latter was provided with an equa- 

 torial of the same size, which had a finder of 0.08 m. 

 aperture, thus givins; the observer two telescopes. 

 The finder had a field of 4°..5, and was used in exam- 

 ining the region in the vicinity of the sun, while the 

 larger instrument was intended to give the position 

 of any strange object that might be noted by means 

 of its position-circle-!. In order to avoid the neces- 

 sity of reading the circles, an attachment was made 

 to both right ascension and declination circles, by 

 which fine marks could he made upon the circles and 

 verniers by the observer's assistants, and the corre- 

 sponding readings determined at leisure. The finder 

 was also furnished with a reticule containing cross- 

 threads, and a position-circle for use in noting the 

 appearance of the corona, to the drawing of which 

 Mr. Trouvelot gave a portion of the time of the total 

 phase. 



The search for intra-mercurial planets was also con- 

 ducted by the aid of photographic apparatus, which 

 Mr. Janssen thus describes: — 



'■ At my order, Mr. Gautier had prepared an equa- 

 torial mounting with an hour-axis two metres long, 

 carrying a strong and large platform, upon which were 

 fastened the following photographic apparatus: a 

 large camera havini; a lens of eight inches (0.21 m. ), 

 made by Darlot, giving a field of 20° to 2.0° (plate of 

 0.40m. by 0.50 m.), and designed for photographing 

 the corona and the region about the sun with refer- 

 ence to the stars there found; a second camera, with 

 a Darlot lens of six inches (0.16m.), giving a uold of 

 2U° to 35° (plate of 0.30 m. by 0.40 m.), for the same 

 purpose; and a very fine apparatus by Steinheil, for 

 studying the corona. A second mounting carried 

 several cameras with lenses of four inches (0.10 m.), 

 giving a great amount of light, and designed to deter- 

 mine by very sensitive plates what are the limits of 

 the corona, — an .apparatus of great light-power, the 

 exposure la-*ting during the whole of totality." 



For spectrum analysis the following apparatus was 

 employed: "a [reflecting] telescope of O.oOm. aper- 

 ture, having a very short focus ( 1 00 m.), and supplied 

 with a direct-vision spectroscope of ten prisms; the 

 slit of the spectroscope could be placed at ilifferent 

 j)osit,ion-angles, and rapidly opened or closed, at the 

 pleasure of the observer. An excellent finder, sup- 

 plied with a reticule, was placed near the spectroscope, 

 and distant from it by such an amount, that, when 

 one eye had fixed upon some point of the corona in 

 the finder, the other could obtain the spectroscopic 

 analysis of this point." There was also attached to 

 this telescope a biquartz polariscope by Pr.izmowski, 

 and a spectroscope for showing Kespighi's rings. A 



