IO4 Chemical and Physical Notes 



easily attained there than in regions more remote from the 

 equator. 



But the ideal conditions of temperature for a first-class 

 physical laboratory are to be found on board a large wooden 

 ship at sea between the tropics. The temperature of the 

 surface-water does not vary by i F. in twenty-four hours. 

 The temperature of a shaded thermometer on deck may 

 vary by two or three, or even more, degrees, according to 

 the greater or less efficiency of the shade, but the true 

 temperature of the air varies as little as does that of tJie sea 

 surface. 



During the three years that the " Challenger " sojourned 

 in tropical seas the writer had daily occasion to notice this 

 fact, and to some extent, though inadequately, to take ad- 

 vantage of it. In particular, a series of experiments was 

 made on the compression of deep-sea thermometers, which 

 required practically absolute uniformity of the temperature 

 of the air. This was found on the main-deck, where the 

 compression apparatus was installed. The main-deck of the 

 ship was protected by the spar-deck, and by an awning above 

 that. It was ventilated by twenty-eight open gun-ports. All 

 passages were made under sail ; therefore, when under way, 

 one side of the ship was always definitely a weather side and 

 the other definitely a lee side, and the gun-ports afforded 

 unobstructed passage to whatever wind was blowing. The 

 main-deck of the " Challenger " was, therefore, a perfect 

 " thermometer screen," and tested in these perfect conditions 

 the air preserved as uniform a temperature as the water. 

 While the compression experiments were being made the 

 variation of temperature during the working part of any one 

 day was not greater than one-tenth of a Centigrade degree. 



It would be impossible on shore to provide such conditions 

 of work. Hence, so far as temperature conditions are con- 

 cerned, and other things being equal, observations of the 

 barometer on board ship in tropical regions are entitled to 

 more weight than those made elsewhere. 



Giving effect to these considerations, we have the 

 following : 



