896 Transactions of the American Institute. 



Batuomkter. 



Dr. A. W. Hall exliibitcdhis bathometer for deep sea soundings. 

 It consists of a tube closed at the top and open below, in which is 

 inserted a graduated rod. As the instrument descends the increasing 

 pressure forces the M'ater up into the tube, compressing the air. 

 The height to which tlie water ascends is known by its wetting a 

 preparation covering the rod and leaving its mark ; thus the depth 

 is recorded. When the instrument strikes the bottom a weight is 

 automatically detached, and the machine rises by means of a float 

 attached to it. In instruments involving the principle of compressing 

 air there is danger of collapse, even at a pressure of five or six tons 

 to the square inch, where the pressure within and \vithout the instru- 

 ment is equalized as in this machine. 



Mr. C. E. Emery suggested that the compressed air would be 

 absorbed bj the water to a greater or less extent, and thus cause 

 inaccuracies in the record. 



Dr. Yanderweyde thought that this difficulty could be overcome 

 by covering the surface of the water in the tube with oil or similar 

 fluid. 



The Aurora Borealis. 

 Dr. P. II. Yanderweyde said that a few weeks ago, the subject 

 of the aurora borealis was under discussion, and a paper Avas then 

 read by Dr. Hall, in which he contended that the aurora borealis is 

 nothing but reflected sunlight. He, therefore, proposed to occupy 

 their attention on that evening with some remarks in opposition to 

 that theory, and in defense of tlie electric theory. That the aurora 

 was the reflection of the sun is and old idea, and abandoned years 

 ao-o. It was again debate'd by the great men of more recent times, 

 and with the same result. The cause of atmospheric electricity is the 

 evaporation of water, water containing salts in solution ; and where 

 Buch water is evaporated one part will be positively and the other nega- 

 tivelv electrified. The vapor rising from the water will be positive, 

 and the water below negative; and when these two electricities are 

 combined, they neutralize each other. They have a tendency to neu- 

 tralize themselves, and so we have the thunder storm. Some clouds 

 may be stronger charged than others, but they influence each other. 

 Our atmos^)here is a non-conductor, and where this is overcome we 

 have a flash of lightning. Sound travels 1,100 feet in a second ; and 

 BO if it take ten seconds ii'om the time a flash of lightning is seen, 



