294 Mr. S. W, Holman on a Simple Method 



least resistance — will combine to carry the luminous particles 

 emitted from the comet in advance of the mass. 



That the change in the direction of a comet's tail should 

 take place with the rapidity and to the extent observed in the 

 case of the enormous appendages of some of these bodies may 

 still appear surprising. But it must be remembered that, in 

 these cases, we can speak only of what we see. The conical 

 mass of gases or vapours extending behind the nucleus of a 

 comet may attain, in the case of the largest of these bodies, 

 to an expansion much greater than the part visible, which may 

 consist only of the parts that receive the strongest impulses 

 from the centre of force; so that when the tail seems to have 

 swung round through an enormous arc in the sky, what has 

 really happened may be only that the line along which the 

 substances forming it become visible may have shifted, in con- 

 sequence of the direction of the impulses proceeding from the 

 head having altered. 



As the time when the most rapid alteration in the direction 

 of the tail of a comet takes place necessarily coincides with 

 that when the expansive action of the sun on the substances 

 emitted from the comet is at its maximum, there must be the 

 less difficulty in admitting the last hypothesis as an explana- 

 tion of this phenomenon. It is the only one, so far as I see, 

 that offers any difficulty in the way of the theory now proposed 

 respecting the tails of comets, which may be summed up in 

 the proposition that, as the incandescence of meteoric bodies 

 proves to us the existence of a widely diffused atmosphere 

 surrounding the earth, so the development of the tails of 

 Comets proves to us the existence of a much more widely dif- 

 fused atmosphere surrounding the sun — both sets of pheno- 

 mena being due to the same cause, namely the resistance of 

 these atmospheres to bodies rapidly passing through them. 

 I am yours &c, 



E. Yansittart Neale. 



15 Portsmouth Street, Manchester, 

 August 21, 1882. 



XXXIII. Simple Method for Calibrating Thermometers, 

 By Silas \V. Holman*! 



THE calibration of a thermometer by most of the methods 

 in ordinary use is a tedious and somewhat difficult opera- 

 tion, and hence often neglected even in important work. For 

 the purpose of supplying a method simple both in observation 

 and computation, and at the same time accurate, the following 

 * From Sillimau's American Journal, Xo, 136, p. 278. 



