XXXV1\ . "TV BOOTES. 



H p. 131. ATniS=fffiuaire, 1842, p. 246. Several physical facts ap- 

 pear to indicate, that when a mass of matter is mechanically reduced to a state 

 of extreme division, if the mass be very small in proportion to the surface, the 

 electric tension may increase sufficiently for the development of light and heat. 

 Experiments with a large concave mirror have not hitherto given any decided 

 proof of the presence of radiant heat in the zodiacal light. (Lettre de M. 

 Matthiessen a M. Arago, Comptes-rendus, T. xvi. 1843, Avril, p. 687.) 



(") p. 132. "What you tell me of the changes of light in the zodiacal 

 light, and of the causes to which, within the tropics, you ascribe such varia- 

 tions, has excited my interest the more, because I have been for a long time past 

 particularly attentive every spring to this phsenomenon in our northern lati- 

 tudes. I, too, have always believed the zodiacal light to rotate ; but I assumed 

 it (contrary to.Poisson's opinion which you communicate to me) to extend 

 the whole way to the sun, increasing rapidly in intensity. The luminous 

 circle which in total eclipses shews itself around the darkened sun, I have sup- 

 posed to be this brightest portion of the zodiacal light. I have satisfied 

 myself that the light is very different in different years, sometimes for several 

 successive years being very bright and extended, and in other years scarcely 

 perceptible. I think I find the first trace of any notice of its existence in a 

 letter from Rothmann to Tycho Brahe. Rothmann remarks, that in spring 

 he has observed the twilight ceased when the sun had descended 24 beneath 

 the horizon. Rothmann must certainly have confounded the disappearance of 

 the zodiacal light in the vapours of the western horizon with the real termina- 

 tion of the evening twilight. I have not myself been able to observe the 

 sudden fluctuations in the light, probably on account of the faintness with 

 which it appears to us in this part of the world. You are certainly right in 

 ascribing the rapid variations in the light of celestial objects, which you have 

 perceived in the climate of the tropics, to changes taking place in our atmos- 

 phere, and especially in its higher regions. This shews itself in a most striking 

 manner in the tails of great comets. Often, and particularly in the clearest 

 weather, pulsations in the tails of comets are seen to commence from the head 

 or nucleus as the lowest part, and to run in one or two seconds through the 

 whole extent of the tail, which in consequence appears to lengthen several 

 degrees, and contract again. That these undulations, which engaged the 

 attention of Robert Hooke, and in later times of Schroter and Chladni, do not 

 take place in the cometary tails themselves, but are produced by our atmos- 

 phere, appears evident if we reflect that the several particles of these cometary 

 tails (which are many millions of miles in length) are at very different distance* 



