WILLIAMSON ON COMETS. 197 



cubic inch of air, compressed as it is on the surface of the earth, it 

 it should be removed from the earth, so far as not to be affected by 

 the attraction of gravitation, would expand itself, by the repelling 

 power of the several particles, so as to fill a globular space, eighteen 

 hundred millions of miles in diameter. Hence it is clear, that if our 

 atmosphere was not retained by the attraction of gravitation, we should 

 soon be forsaken by it, and perish. Now, if every particle of air is thus 

 repellant of every other particle, it is clear that an immense body of 

 air, compressed by gravitation round the sun, must operate with great 

 force in repelling another body that is attached to a comet, and it is 

 equally clear that it must repel that atmosphere with the greater force 

 the nearer they come together. 



If it should be inquired, whether the cometarians, in their aphelion, 

 would not be oppressed by the excessive weight of their atmosphere ; 

 and whether they would have sufficient light to pursue their necessary 

 occupations ? To such inquiries we reply, that though with us, as above 

 stated, the air presses upon every square foot of a man's body, with a 

 pressure equal to the weight of two thousand one hundred and sixty 

 pounds, we are not oppressed by such weight, because the internal and 

 external pressures are alike. On the subject of light, we know that 

 the pupil of the eye may be so constructed as to fit itself to a very 

 strong, or a very moderate, light. Nor have we any reason to think 

 that the cometarians suffer on that head. Dr. Halley has calculated 

 that the distance of the comet of 1680, was to the earth's distance as 

 one hundred and thirty-eight to one. But Smith, in his Optics, has 

 calculated that the light of the sun is to the light of the full moon as 

 ninety thousand to one. Now if we divide ninety thousand by the 

 square of one hundred and thirty-eight, we shall find that the cometa- 

 rians, in their aphelion, enjoy a light nearly five times as strong as the 

 light of our full moon ; such a light must be quite sufficient. 



