Life of the Waters 367 



correction can be made for this error however if the tem- 

 perature difference be known, and so in one of the best 

 types of instrument an accessory thermometer is mounted 

 alongside the principal one, and thus the difference in tem- 

 perature between the water and the air at the moment of 

 reading can be taken and the correction applied. 



The reversal of the deep sea thermometer is effected either 

 by means of a small propeller which commences to revolve at 

 the moment of raising the thermometer, or more usually by 

 means of a weight or "messenger" which is dropped down 

 the line and trips a catch, releasing a spring and upsetting 

 the thermometer. 



An . estimation of the light penetrating the ocean may be 

 made by lowering a white, or variously colored disk and 

 noting the point at which it disappears, and vice versa by 

 raising it from lower depths and noting the depth at which 

 it can first be seen. By taking the mean of these two read- 

 ings, and comparing it with some empirical standard, an 

 estimate may be made of the transparency of the water. 

 Electric lights are also employed for this purpose. A better 

 method is the use of a photometer. Various types of these 

 are employed, mainly designed on the principle of exposing 

 photographic plates or films contained in a water-tight holder 

 for varying periods of time and noting the density of the 

 exposure after development. By comparing the density of 

 these exposures with a standard set made in the air for 

 different lengths of time, and taking into account the angle 

 of the sun's rays with the horizontal and the turbidity and 

 color of the water, an estimate may be obtained of the in- 

 tensity of the total illumination at various depths. If it is 

 desired to determine the relative penetration of different 

 rays of light (red, green, blue), plates of colored glass, with 

 a known capacity for absorbing (and therefore cutting off 

 from the plate or film) certain rays and admitting their 

 complementary rays, may be placed over it. Such an appa- 

 ratus is crude at best, and it remains for the investigator of 

 the future to devise a machine presumably of electrical con- 

 struction, which will give an accurate determination of the 

 light at different depths in the sea. 2a Even with the 

 crude apparatus thus far devised however many interest- 

 ing results have been obtained. The depth to which light 

 penetrates the sea has already been mentioned. It has also 

 been found that night-time comes much sooner for the chil- 

 dren of the sea than for those of the air, for when the sun 

 is yet many degrees above the horizon, the surface of the 

 water acts as a mirror and totally reflects its rays. The 



2a Such apparatus has been devised, but has not yet come into general 

 use. 



