NATURAL PHILOSOPHY. 147 



moans of the electric light thrown through water-ti^ht lens win- 



^ C^ ^5 



dows upon the objects to.be photographed. The value of this 

 invention in submarine surveying is obvious. 



Pkotomicograpky. For a very interesting paper on this subject, 

 see " Popular Science Review" for Januaiy, 1867. 



Photographic Light. Mr. Sayers proposes the following: ni- 

 trate of potash in powder, and well dried, 24 grammes; flour of 

 sulphur, 7 grammes; red sulphuret of arsenic, 7 grammes. These 

 three ingredients being well ground together, the mixture, on 

 being ignited, will yield a most powerful photogenic light; 200 

 grammes of the compound will make the light last half a minute. 

 The cost of the mixture is not more than 16 to 20 cents per kilo- 

 gramme, which would last two minutes and a half, while light 

 from magnesium wire costs about Is. per minute. 



THERMOGRAPH AND BAROGRAPH. 



In the thermograph used by the meteorological committee of 

 the British Association, as described at the meeting at Dundee 

 in September, 1867, an air-speck, formed by a break in the mer- 

 curial column of a thermometer, allows the light of a gas lamp to 

 pass 'through it, producing an image which is obtained on a re- 

 volving cylinder covered with photographic paper. As the cylin- 

 der revolves once in 48 hours, and as the thermometric column 

 rises and falls, these motions delineate a curve, by means of which 

 the temperature of the thermometer is denoted from moment to 

 moment. There would be but one curve if there were only one ther- 

 mometer; in practice there are two, the dry and wet bulb, one 

 registering the temperature, the other the amount of evaporation. 

 In this instrument the simultaneous records of the two thermome- 

 ters are obtained, one under the other, on the same sheet of paper, 

 the under curve denoting the readings of the wet bulb ther- 

 mometer, and the upper those of the dry bulb. 



By an arrangement of Mr. Beckley, the light is cut off from the 

 sensitive paper for four minutes every two hours. A small break 

 is thus produced every two hours on -each curve, by means of 

 which the time of any phenomenon may be easily ascertained. 

 By drawing lines through the simultaneous breaks of the wet and 

 dry bulb curves, a series of lines is obtained perpendicular to the 

 direction of motion of the cylinder, which serves the purposes of a 

 zero-line. 



The arrangement for cutting off the light every two hours is 

 also introduced in the barograph used by the committee. In this 

 instrument the curve denotes an uncorrected barometer; the zero- 

 line is not a straight line, but is formed by the interception of the 

 light from the cylinder by a stop, which, by means of a lever 

 arrangement, rises and falls with temperature as much as the 

 barometric column rises and falls from the same cause ; that is to, 

 say, in order to find the true height of the barometer, we measure 

 between the zero-line and the line denoting the top of the uncor- 

 rected column, since, when the top of the column rises or falls 

 through temperature, the zero-line rises or falls just as much. 



