NATURAL PHILOSOPHY. 151 



two or three notes, was conveyed to one ear, while the other was 

 stopped. He convinced himself that an impression of sound could 

 arrive at the ear in this manner only, and yet all the notes were audi- 

 ble at the same time at all the different points. Hence Duhamel con- 

 cludes, that if the oscillatory motion of one point be decomposed into 

 several others, the ear is affected in the same manner, whether the 

 component movements emanate simultaneously from several neigh- 

 bouring points, or from one point only. Liebig's Annual Report. 



MIRAGE ON LAKE SUPERIOR. 



DK. CHARLES T. JACKSON communicated to the American Scien- 

 tific Association, during its session at Cambridge, an interesting paper 

 entitled " Observations on the Mirage seen on Lake Superior in July 

 and August, 1847." 



Si The phenomena of mirage have at all times excited the wonder 

 and admiration of mankind, and have been fruitful in strange super- 

 stitious legends. Even those most versed in the causes of natural 

 phenomena cannot fail to be strongly impressed with the magnificent 

 phenomena of mirage on the north shore of Lake Superior, and the 

 philosophical mind delights in being able there to observe the causes 

 which produce this marvellous effect. I know not whether the sea- 

 son when I had the opportunity for making my observations was one 

 remarkable for the frequency of mirage, but it is certain that, for many 

 successive days, the phenomena were presenting themselves in rapid 

 succession along the northern coast of Lake Superior, opposite to Isle 

 Royale, and on the coast of the island itself, in the bays which so 

 deeply indent its shoves. At Rock Harbour, on several occasions, I 

 observed the little islands and points on its outskirts most perfectly 

 represented, with inverted pictures of their entire forms hanging over 

 their summits, the images of the spruce and other trees which crown 

 them being seen with beautiful distinctness directly over their terres- 

 trial originals, while the picture of a little skiff was one day seen 

 represented beside the phantom island, the boatman in the sky appear- 

 ing to raw his batteau as unconcernedly as his original on the bosom 

 of the Lake. 



" On the 27th of July we saw Keweenaw Point in mirage. It is 40 

 miles distant from this place, and bears E. N. E. from SeoviPs Point 

 on Isle Royale. The most wonderful mirage was observed from the 

 north coast of Isle Royale, while we were coasting along from the 

 eastern to the western end of the island. For several days in succes- 

 sion, we had almost hourly magnificent repetitions of these curious 

 phenomena. Thunder Cape, 15 miles distant to the north, a lofty 

 mural precipice, said to be 1,300 feet high, and rising directly from 

 the lake, presents the form of an irregular truncated pyramid. By 

 the phenomena of mirage it suddenly changes its form into a huge 

 anvil, sending out a long horn to the right, while a dark black mass 

 rises behind it which might be represented as old Vulcan himself. 

 This singular phenomenon attracted much attention, and on observing 

 with care, I found that the horn of the anvil was the image of the 



