• KNOWLEDGE • 



215 



to one five-hundredth part of the terrestrial atmosphere, 

 for the mass of the moon is less that one-eightieth part of 

 the earth's mass, so that the weight of a body at the 

 moon's surface is less than one-fifth of the weight it would 

 have upon the earth's surface. And an atmosphere like 

 our own would, if removed to the moon (the temperature 

 remaining the same), Le more loosely packed, and would 

 extend to a much greater height above the surface. 



There is another method by which the existence of a 

 lunar atmosphere may possibly be detected. Besides 

 bending rays of light, the earth's atmosphere absorbs cer- 

 tain wave lengths more than others, so that in the spectrum 

 of sunlight which has passed through the lower strata of the 

 atmosphere for some distance, as at sunrise or sunset, there 

 are certain lines which are not to be found in the spectrum 

 of a high sun. M. Thollon, who was one of the observers 

 of the total eclipse at Sohag, has spent many years in 

 making a great map of the solar spectrum, in which he has 

 laid down the position and relative intensities of the solar 

 lines, and he has also mapped down the places of many 

 hundreds of atmospheric lines which are proved not to 

 belong to the true solar spectrum, because they grow more 

 intense as the sun sinks towards the horizon, and vary 

 wth the amount of aqueous vapour present in the atmo- 

 sphere. 



The instrument with which M. Thollon has made his 

 observations is a bisulphide of carbon spectroscope, giving 

 a dispersion equivalent to thirty-one prisms of 60° of 

 ordinarily dense flint glass, and giving a much brighter 

 spectrum and far better definition than could be obtained 

 with such a cumbersome train of prisms, if it were possible 

 to mount them and keep them in adjustment. M. Thollon 

 has in a very simple manner contrived to make the solar 

 rays pass three times through a system of prisms and half- 

 prisms, so that the eye-piece of the viewing telescope and 

 the slit remain fixed, while by a motion of the prisms, 

 different parts of the spectrum can be brought into view. 

 M. Thollon has made with this instrument a map of the 

 solar spectrum 49 feet long, showing more than 4,000 

 lines. 



■*36 -^S: ^3S -^39 -^iO ■*41 '•rZ ■*43 -*M -*<i -^46 



_L ■ 



The ■wood-cut, copied from one of M. Thollon's drawings, 

 represents the well-known B group, which lies towards the 

 red end of the spectrum. It is a group which is modified 

 in a most striking manner by atmospheric absorption. 



During the total solar eclipse of July, 1878, Professor 

 C. A. Young examined this region of the solar spectrum 

 with the slit, of his spectroscope placed upon the image of 

 tlic solar crescent, so that light from the sun's disc entered 

 one half of the slit while the other half of the slit was upon 

 the image of the dark moon. On looking into the eye-piece 

 of his spectroscope a brilliant solar spectrum was seen in 

 the half of the field corresponding to the half of the slit 



which was upon the sun's disc, while the other half of the 

 field appeared quite black. The division between the two 

 halves of the field was exceedingly sharp. There was no 

 gradual dimming of the solar spectrum close to the moon's 

 limb, and Professor Young was not able to recognise any 

 of the well-marked terrestrial absorption lines, or any new 

 lines in the solar spectrum, though he spent some time ia 

 carefully examining the region close to the moon's limb. 



Professor Young's observation was made with a grating 

 spectroscope of 17,300 lines to the inch, which in the 

 spectrum observed gave a dispersion about equal to the 

 dispersion of M. Thollon's liquid spectroscope; but the 

 amount of light and the sharp definition obtainable with 

 Professor Y''oung's instrument was not equal to that given 

 by M. Thollon's instrument. 



It will be seen that the light which enters the slit 

 close to the moon's limb must, if there be a lunar atmo- 

 sphere, have passed through the densest portions of it ; 

 but although the air of Colorado, where the observation 

 was made, was very dry (and the sun was at a consider- 

 able altitude, more than 41° above the horizon, so that 

 there would be very little absorption taking place in 

 the earth's atmosphere, and the lunar absorption 

 lines, if any, ought by contrast to have been well 

 shown), Professor Young was unable to detect any trace 

 of absorption. 1 was observing near to him, and he kindly 

 gave me an opportunity of looking through his instrument. 

 I was much struck by the sudden transition from the 

 ordinary solar spectrum to utter darkness, a transition 

 totally unlike that which is observed when the slit lies 

 across the nucleus of a sun spot. The edge of the spectrum 

 was much more strikingly abrupt and sharply defined than 

 the edge of the spectrum which corresponds to the sun'3 

 limb when the slit is placed radially on the uneclipsed sun 

 so as to project beyond the solar disc, and the dark lines 

 of the solar spectrum were sharply cut off, apparently 

 without alteration. No evidence, therefore, was obtained 

 by Professor Young tending to show that any absorption 

 of recognisable amount takes place at the lunar limb. 

 In our next article we will examine the evidence which 

 was obtained by MM. Thollon and Trepied during the 

 recent total eclipse which they observed in the dry air of 

 Upper Egypt. 



(To he continued.) 



A Super Sensitive Thermometer. — Since the days when 

 Mr. Edison brought out his micro-tasimeter, which proved 

 so sensitive to heat, until now, we have had no instrument 

 devised for measuring extremely delicate changes of tem- 

 perature. Such an apparatus has, however, been recently 

 devised by M. Michelson, and brought, at least in its ex- 

 perimental form, Viefore the French Physical Society. It 

 is based on the principle of bimetallic thermometers, but 

 ebonite or hard caoutchouc is chosen instead of one metal. 

 Hard rubber is ten times more dilatable than platinum 

 under heat, and a spring composed of platinum on one side 

 and ebonite on the other will curve under the least increase 

 of temperature. At the extremity of the spring is fixed a 

 small glass stem, forming an elbowed lever, which abuts 

 against a light mirror suspended by a silk fibre. AVhen the 

 spring curves or straightens, the mirror is dcfiected, and a 

 ray of light from a lamp reflected from its surface to a scale 

 moves up or down the divisions of the scale. By giving t« 

 the spring and lever a relatively great length, this instru- 

 ment can be made very sensitive, and the inventor hopes 

 to be able to measure the thousandth rf a degree Centi- 

 grade. — Engineering. 



