4 CHARACTER OF THE SKY IN VIRGINIA. 



of Virginia, upon the same parallel of latitude as Tunis and the more northerly African 

 kingdoms I thought the situation too favourable to suffer such an opportunity to pass 



i 



without endeavouring to gain some decisive information on this contested point. 



4. The sky of that part of Virginia is not, however, so bright as might be expected. 

 When unclouded, it is of a clearer blue than the sky of England, and, I think, rather 

 darker, approaching, by several shades, nearer to the tint of Prussian blue than that 

 azure colour which it has in the latter country. During a residence of twelve months, 

 1 saw it twice of that intense complexion which they say it has in the tropical regions. 

 The moon appeared globose, and her centre to bulge out like a ball, and the planet 

 Venus might be fancied to exhibit a well-formed crescent. The air passes from great 

 moisture to dryness with rapidity ; a good barometer is seldom at rest. The tempera- 

 ture of the sunbeam is sometimes 132 F., but the siphon barometer seldom varies more 

 than one inch. Clouds form and rise with great rapidity, the seasons are uncertain, 

 and the atmosphere is often deformed with a mistiness that obscures distinct vision. 

 But the dense clouds that gather in the west of an evening emulate the beauty of a 

 sunset on the Atlantic Ocean. 



5. I am tlms particular in describing the state of the atmosphere in which these ex- 

 periments were made, because much stress has been laid upon that circumstance, as ma- 

 terially affecting the results of magnetic action of the solar ray. And, as the experiments 

 now to be detailed lead to a negative decision, it is well to explain under what circum- 

 stances they were performed, that those who have come to an opposite conclusion may 

 have an opportunity of pointing out whether it is really owing to the state of the atmo- 

 sphere. 



6. In the year 1824, Mr. Christie found that a needle six inches long, contained in 

 a brass compass box with a glass cover, suspended by a hair, and made to vibrate, al- 

 ternately shaded and exposed to the sun, came to rest much sooner in the latter than 

 in the former case. That this was not occasioned by an increase of temperature, was 

 proved by the needle vibrating more rapidly when its temperature was raised by other 

 means. 



7. In repeating this experiment, I very quickly found that it depended, in a great 

 measure, on the nature of the suspension of the needle, and its position with respect to 

 the incident light, what results would be obtained. If the needle was suspended on a 

 point or by a thread, without torsion, both the time and the number of vibrations were 

 the same, whether the needle was exposed to the sunbeam or not. But, if the needle 

 was suspended by a hair or other organic substance, having torsion, the sunbeam would 

 occasion a degree of twist in the hair on its first exposure to light ;*and if the direc- 

 tion of that twist happened to coincide with the direction of the needle's motion, of 

 course the momentum of the needle was increased, and the vibrations continued longer. 

 A needle which vibrated forty-four times in one minute, would occasionally, owing to 

 this cause, vibrate nearly forty-six when suspended by a hair ; but if by a silk fibre, its 

 vibrations were always forty-four, the first arc of vibration being in every instance 40. 

 That this action was due to a twist of the hair, as a hygrouietric body, I ascertained 

 by means of a simple arrangement. To a brass cross-piece, A (fig. 1, pi. 1), supported 



