246 Causes of the Aurora BoreaUs. 



5. None of the locomotive properties of the aurora are more uni- 

 form, than its inclination to form an arch ; for however variable its 

 figures and motions, an arch, although sometimes irregular, generally 

 circumscribes the phenomenon. This is formed by the refraction 

 over the spherical figure of the earth. 



6. The splendor, intensity and frequency of the phenomenon in- 

 creases from the temperate regions to the Arctic circle. The mag- 

 nificent aurora of Sept. 1827, was noticed through New York, New 

 England, the Canadas, and the north of Europe. In Quebec, N. 

 Lat. 46°, it was much more brilliant than in New York, Lat. 41°, 

 while at Baltimore, Lat. 38°, it was but a faded resemblance of that 

 seen in the northern States, and south of Maryland it was not no- 

 ticed, probably not seen at all.* This coincides with the laws of 

 refracted light, for the power of refraction increases from the equa- 

 tor to the pole. Before the commencement of the polar summer, 

 the twilight continues till midnight in the Arctic regions ; while un- 

 der the tropics there is very little refraction, no evening twilight or 

 morning, dawn. 



7. As the beautiful prismatic hues, which often increase the splen- 

 dor of the aurora, must be referred to the reflection of the light upon 

 the watery particles contained in the vapor, so they also indicate the 

 proportion of water diffused through the strata, and the angle at which 

 the light is reflected. 



Effects upon the Magnetic JYeedle. 



It has been extensively admitted that the aurora influences the 

 magnetic needle, from which it has been inferred that magnetism is 

 in some way a cause or an effect of the phenomenon. But the ef- 

 fect upon the needle does not appear to be well ascertained. Capt. 

 Parry, assisted by the scientific gentlemen of his expedition, made 

 the strictest observation in order to establish the amount of its influ- 



twilight, which is described as being often of the richest crimson, purple and orange, 

 of great depth and splendor. Baron Humboldt states the maximum of the barome- 

 ter at 28 inches at the foot of the Peak of Teneriffe, lat. 28°. At Winter Island, hit. 

 66°, it was 30 inches. Prof. Olmsted's table makes the maximum of the barome- 

 ter at New Haven 30 inches in 1827. I have emploj^ed round numbers, although 

 there were perhaps a few lines more than 30 inches in each case ; but the difference 

 of two inches in 33° of latitude, seems, as regards increase of den^iity, altogetjjer in- 

 sufficient to produce the amount of atmospheric refraction existing at the pole. 

 * American Journal of Science, Vol. xiv. 



