NATURAL PHILOSOPHY. 151 



He erects a frame-work on the locality of danger, in which he places a com- 

 bination of simple mechanism for raising and lowering signals flags or balls 

 for the daytime, and colored lights for the night. The mechanism is operated 

 by a float resting in the water. As the tide rises and falls the machinery 

 moves and the signals change. Thus, there may be a signal for each foot of 

 depth; when the water is two feet deep, two signals will be shown; as 

 soon as the tide has risen another foot, three signals will be exhibited and 

 so on, vice versa. Scientific American. 



On the detection and measurement of Atmospheric Electricity ~by the Photo- 

 barograph and Thermograph. Photography has already rendered considerable 

 aid to science, and some results brought before the British Association by Mr. 

 Johnson, Radcliffe Observer, Oxford, furnish an example of this. On examin- 

 ing and comparing the registrations of the thermometer and barometer certain 

 peculiarities presented themselves which indicate a curious connection between 

 the course of these instruments and the state of the weather. The line which 

 indicates this course is sometimes serrated, sometimes even and continuous ; 

 and these appearances correspond to certain determinate states of the weather. 

 The most remarkable result is a sudden change of the height of the barometric 

 column, which takes place simultaneously with the occurrence of a peal of 

 thunder: a contemporaneous effect was produced upon the thermometer. 



GIST THE AUEOEA BOREALIS. 



At the last meeting of the British Association, Sir John Eoss, presented a 

 communication, in support of a theory respecting the origin of the Aurora 

 Borealis, which he first promulgated some years ago. Sir John Ross says : 

 " It having occurred to me that, if my theory was true, namely, 'that the phe- 

 nomena of the aurora borealis were occasioned by the action of the sun, when 

 below the pole, on the surrounding masses of colored ice, by its rays being 

 reflected from the points of incidence to clouds above the pole which were 

 before invisible,' the phenomena might be artificially produced ; to accomplish 

 this, I placed a powerful lamp to represent the sun, having a lens, at the focal 

 distance of which I placed a rectified terrestial globe, on which bruised glass, 

 of the various colors we have seen in Baffin's Bay, was placed, to represent the 

 colored icebergs we had seen in that locality, while the space between Green- 

 land and Spitzbergen was left blank, to represent the sea. To represent the 

 clouds above the pole, which were to receive the refracted rays, I applied a hot 

 iron to a sponge ; and, by giving the globe a regular diurnal motion, I produced 

 the phenomena vulgarly called " The Merry Dancers," and every other appear- 

 ance, exactly as seen in the natural sky, while it disappeared as the globe turn- 

 ed, as being the part representing the sea to the points of incidence. In cor- 

 roboration of my theory, I have to remark that, during my last voyage to the 

 Arctic Regions (1850-1), we never, among the numerous icebergs, saw any that 

 were colored, but all were a yellowish white ; and during the following winter, 

 the aurora was exactly the same color ; and, when that part of the globe was 

 covered with bruised glass of that color, the phenomena produced in my exper- 



