APPENDIX TO MEMOIR OF PELTIER. 193 



From tlie above, it is seen tliat tlio maximum of coloration is always found in 

 the extraordinary image, with the sun at 90°. Now, it is evident that this should 

 be so, since the angle of maximum polarization for the air is precisely at 4o°. 



If we look with the objective tube of the polarimeter towards a point of the 

 heavens situated outside of the o})tical meridian and equator, the blue of the 

 extraordinary image is altered ; it becomes violet, or else green, according to 

 the direction of the displacement. This change in the tint of the images indi- 

 cates an equivalent change in the plane of the reflector or polarizer. To regain 

 the blue, it is necessary to turn the bi-refracting prism by an angular quantity 

 equal to the supposed angular deviation of the reflector, with a view to replacing 

 the principal section of the prism in the same relation that it had with the plane 

 of the polarized ray befoi-e this deviation of the reflector; we therefore turn the 

 ocular tube which bears the bi-refracting prism, until we shall have recovered 

 the blue ; then we carefully note the number of degrees by which it has been 

 turned, for it is this notation which gives the position of the plane of polariza- 

 tion in the point of the atmosphere which w'e may be studying. Unluckily, the 

 rotation which has been communicated to the ocular to restore the blue of the 

 extraordinary image, has at the same time destroyed the blue of the ordinary 

 image which proceeds from the pile and from the other lamina of quartz — that is 

 to say, the blue which is to serve as a point of comparison ; it was requisite, 

 therefore, to find the means of reproducing the normal blae of the ordinary 

 image. 



To attain this result, Peltier covered the cap of the left and his lamina of 

 quartz with another cap, turning with easy friction. In this new cap he set a 

 lamina of mica of a thickness sufficient to restore to the image its normal blue by 

 turning the cap on itself, and placing by this means the principal section of the 

 lamina of mica in the plane necessary to obtain this restoration of the blue. 



V. — Meteorology. 



Intwdiicfion. — Astronomical and meteorological phenomena are, beyond ques- 

 tion, the first which must have attracted the attention of man. The diurnal 

 movement of the sun, its annual movement and the periodical return of the 

 seasons, must have so much the more interested him as they bore directly on his 

 existence and his material well-being. On the other hand, the astounding spec- 

 tacle of storms, the lightning and the thunder, could as little fail strongly to 

 impress his imagination. Everytliing, therefore, would lead us to conclude 

 that, from the earliest times, mankind have been seriously occupied with the 

 study of the different phenomena of astronomy and meteorology. 



But if these two sciences were born at the same time, they are far from liaving 

 made the same progress. Astronomy has long ago attained a certainty so great 

 that it may be considered in this respect the first of all the sciences of observa- 

 tion ; meteorology, on the contrary, is still in its infancy. The reason of this 

 difference is easily comprehended. The movements of the heavenly bodies arc? 

 sulijected to a small number of very simple laws, always identical ; meteorolo- 

 gical phenomena, on the contrary, are generated l)y the action of a host of dif- 

 ferent causes, all widely diverse, and highly variable as to their nature, their 

 mode of action, their power and their mutual influence. But this is not all : to 

 arrive at the point it has attained, astronomy has had to ask little succor from 

 the other sciences ; it has, in some sort, had need only of direct observation for 

 the registration of facts, and of mathematics for their co-ordination and the 

 deduction of consequences. It is not so with meteorology, for meteorology is 

 most frequently only the application of the different law's of jdiysics to a par- 

 ticular class of phenomena, and could not exist in an independent manner ; 

 meteorology, tlierefore, could make no real progress until other sciences, and 

 especially physics, were sufficiently advanced to constitute a satisfactory body of 

 13 s 



