302 THE VOYAGE OF H.M.S. CHALLENGER. 



Temperature. — The dry and wet bulb thermometers were, in the early part of the 

 voyage, suspended in the small screen provided by the Meteorological Department, 

 which was fastened to the after upright of the steering wheel, under the pilotage bridge. 

 As this screen was too small to contain a maximum and minimum thermometer in addi- 

 tion to the wet and dry bulb instruments, a larger one was constructed and placed 

 against the ship's side, under the shade of the pilotage bridge. The thermometers were 

 suspended in it on the 17th April 1873. The thermometers were separated from the 

 ship's side by wooden battens about three inches in thickness. The maximum and 

 minimum thermometers were suspended from the same battens. The instruments in use 

 were occasionally tested for error of zero points (in melting ice) and also compared with 

 a standard thermometer by Geisler. Their indications were seldom in error to the 

 extent of 0°"3. 



Humidity. — The relative humidity has been deduced from the Hygrometrieal tables 

 of J. Glaisher, Esq., F.R.S., 6th edition. 



Sea-Surface. — The temperature of the sea-surface was obtained by drawing a bucket 

 (if water from over the side of the ship, and immersing a thermometer in it. Care was 

 taken that this water should be obtained from a position sufficiently far forward to be 

 clear of the discharge -pipes from the engines, &c. 



When in the region of ocean stream currents the temperature of the surface water 

 was taken at hourly, or half-hourly intervals. In the tables, to avoid crowding the figures, 

 the highest or lowest reading for the two-hourly intervals only is given. 



Clouds. — The amount registered was in all cases estimated, 10 indicating a sky 

 completely overcast, and a clear sky ; the description of cloud is that issued by the 

 Meteorological department of the Board of Trade, and illustrated in the accompanying 

 plate. 



Upper Clouds. 



Fig. 1. Cirrus. — This is the very lofty cloud which looks like hair, thread, or 

 feathers, and is often called " Mare's tails." It frequently moves in a direction different 

 from that of the wind at the earth's surface. 



Fig. 2. Cirro-cumulus. — This is also a high cloud ; it differs from the cirrus 

 in being more globular in form, as it consists generally of small detached rounded masses, 

 like a flock of sheep lying down, or like the markings on a mackerel, whence the name 

 " Mackerel Sky." It is sometimes seen at lower levels, when it may be difficult to dis- 

 tinguish it from small cumuli. 



Fig. 3. Cirro-stratus. — This resembles the stratus in being a sheet or layer of 

 uniform thickness, but it belongs to the higher regions of the atmosphere. 



Whenever there is a doubt the observer had best enter the cloud either as " high 



