METEOROLOGICAL INSTRUMENTS. 227 



in the Report of the British Association, 1849, p. 1 1. It measures 

 the electricity by the displacement of a delicately suspended 

 magnetic needle out of the magnetic meridian. 



OZONE. 



The great difficulty in the measurement of this element is to 

 secure a chemical reaction which shall be an infallible indication 

 of the presence of ozone in the air, as the action of light, &c., or 

 of oxidizing agencies independent of ozone, has been generally 

 found to disturb the correctness of the indications of the test 

 papers. 



Schonbein proposed paper soaked in iodide of starch. 



SPECIFIC GRAVITY OF WATER. 



Instruments for measuring this element are termed Hydrometers. 

 Those used at sea consist of a small glass bulb loaded with shot 

 or mercury. Above this a cylindrical bulb is blown, and the 

 upper portion of the tube, which is retained as a slender stem, is 

 graduated. The depth to which the instrument sinks in water is 

 shown by the scale, and the specific gravity is read off, at the level 

 of the water. 



MISCELLANEOUS INSTRUMENTS. 



Among these may be specially mentioned the automatic light- 

 registering apparatus of Professor Roscoe, which is exhibited, as 

 well as a smaller form of the instrument by Captain Abney. The 

 principle of the method consists in the fact that the depth ef 

 colour produced on chloride of silver is proportional to the 

 intensity of the light multiplied by the duration of the time of 

 exposure. 



In addition to actual instruments, several exhibitors have fur- 

 nished charts and diagrams showing the results at which they 

 have arrived on the progress of their investigations. The most 



Q 2 



