H4 PLEASANT WAYS IN SCIENCE. 



carry them very close to our sun. Newton's comet, with its 

 tail a hundred millions of miles in length, all but grazed the 

 sun's globe. The comet of 1843, whose tail, says Sir John 

 Herschel, ' stretched half-way across the sky,' must actually 

 have grazed the sun, though but lightly, for its nucleus was 

 within 80,000 miles of his surface, and its head was more 

 than 160,000 miles in diameter. And these are only two 

 simong the few comets whose paths are known. At any 

 time we might be visited by a comet mightier than either, 

 travelling in an orbit intersecting the sun's surface, followed 

 by nights of meteoric masses enormous in size and many in 

 number, which, falling on the sun's globe with enormous 

 velocity corresponding to their vast orbital range and their 

 near approach to the sun a velocity of some 360 miles per 

 second would, beyond all doubt, excite his whole frame, 

 and especially his surface regions, to a degree of heat far 

 exceeding what he now emits." 



This theory corresponds far better also with observed 

 facts than the theory of Meyer and Klein, in other respects 

 than simply in antecedent probability. It can easily be 

 shown that if a planet fell upon a sun in such sort as to 

 become part of his mass, or if a nebula in a state of intense 

 heat excited the whole frame of a star to a similar degree 

 of heat, the effects would be of longer duration than the 

 observed accession of heat and light in the case of all the 

 so-called "new stars." It has been calculated by Mr. Croll 

 (the well-known mathematician to whom we owe the most 

 complete investigations yet made into the effect of the 

 varying eccentricity of the earth's orbit on the climate of the 

 earth) that if two suns, each equal in mass to one-half of 

 our sun, came into collision with a velocity of 476 miles per 

 second, light and heat would be produced which would 

 cover the present rate of the sun's radiation for fifty million 

 years. Now although it certainly does not follow from this 

 that such a collision would result in the steady emission of 

 so much light and heat as our sun gives out, for a period of 

 fifty million years, but is, on the contrary, certain that there 



