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Some mechanical Inquiries regarding the Formation of the 

 Tails of Comets. By Dr Lehmann of Berlin *. 



i\ UMERous attempts have been made to explain the formation 

 of the tails of comets. The extraordinary aspect of those bo- 

 dies, by exciting the imagination of natural philosophers, has 

 often drawn them into hypotheses which have run wide of all 

 the known laws of nature. L propose to inquire here, if the 

 form of the tails of comets, and their changes, may not be ex- 

 plained by means of known powers and mechanical laws only, in 

 the same manner as the flux and reflux of the sea are explained 

 by means of gravitation alone. 



Comets do not differ essentially from planets, with respect to 

 their motions, but the eccentricity of the orbit which they de- 

 scribe is much more considerable than that of the orbit of the 

 planets, so that their course is accomplished in a curve, which 

 differs little from a parabola or a hyperbola^ The planets on 

 which we can observe spots, turn at the same time upon their axis 

 as the earth does. The satellites in their motion always present, 

 like the moon, one and the same side to the planet round which 

 they move, the time of their rotation being the same as that 

 of their circulation round the planet. That this agreement is 

 not an effect of chance, but must have resulted, in the case of 

 our moon, from the circumstance that its mass is larger toward 

 the hemisphere which it presents to us -f, is what has been plain- 

 ly demonstrated by the celebrated La Place, in his Mecanique 

 Celeste (L. V. PI 2..)t. 



If we return to the comets, we shall see that two cases may 

 present themselves, with respect to their revolution round an 

 axis. This revolution is performed, either like that of the planets, 

 in such a manner as that they present all the parts of their sur- 

 face in succession to the sun, or, like that of the satellites^ in 

 such a manner as that they always turn the same hemisphere 

 toward that star. It is demonstrated upon mechanical princi- 



* Astronom. Jahrbuch, Berlin, 1826 ; and Biblioth Universelle, Mars 18^6. 



f In consequence of the earth's attraction, and of the circumstance that the 

 mass of the moon must have been originally fluid. 



X It being understood that, at the commencement, the times of these two mo- 

 tions did not differ in any considerable quantity. This explanation is originally 

 due to La Grange. 



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