CHEMICAL AND PHYSICAL NOTES. 151 
classical cellars of invariable temperature. What could be artificially 
provided centuries ago can be provided now. For a central national 
standardising institution such a chamber is indispensable, if its 
certificates are to have the value which ought unquestionably to 
belong to them. 
In tropical, and still more in equatorial regions, where the 
diurnal variation of temperature is small and its rate slow, uni- 
formity of temperature, sufficient for a first-class meteorological 
station, is easily obtained in any well-constructed building, and con- 
sequently perfection is more 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 of a’ large wooden ship at sea 
between the tropics. The temperature of the surface-water does not 
vary by 1° Fahr. 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 vartes as little as does that of the 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 advantage 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 blow- 
ing. 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 com- 
pression 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 concerned, and 
other things being equal, observations of the barometer on board ship 
in tropical regions are entitled to more weight than those made else- 
where. 
Giving effect to these considerations, we have the following— 
