2 
Fune 11, 1885] 
bottom. The mercury remained constant on the zero 
line. The temperature of the air was 25° C." 
Temperature of the Water.—The water was collected 
in a small bucket, well clear of the side of the ship, and 
on the opposite side from that through which the con- 
densing water of the engine is discharged. Its tempera- 
ture was determined as soon as the sample was brought 
on board. 
As the ship left the Channel in the middle of winter, 
and proceeded nearly due south, the temperature of the 
water rose rapidly at first. Observations were begun on 
January 21, in lat. 34° N., and between this latitude and 
lat. 10° N. the rate of rise was very steady, averaging 
0°36° C. per degree of latitude. From lat. 5° N. to 15° S. 
the temperature is very uniform and high, averaging 
26°86° C. After passing lat. 15° S. the temperature falls, 
and begins to show greater variations, as the shallow water 
on the Abrothes Bank is approached. The average tem- 
perature of the water over this bank was 25°56° C. After 
passing Cape Frio, and between the parallels of 25° and 
30° of south latitude, the variations of temperature are 
considerable and often abrupt ; the maximum observed 
in this part was 267 C., and the minimum 24°3 C. As 
the higher temperature generally accompanies a greater 
salinity, it is probable that these variations are due, not 
to any terrestrial source, such as large rivers, but to an 
oceanic cause, the less salt and colder water of the deeper 
ocean strata being thrown up against the coast, and mix- 
ing imperfectly with the hot and dense surface-water. In 
lat 30° S. the influence of the River Plate makes itself 
distinctly felt by a general rapid fall of temperature. As 
the ship got into soundings, with the change in colour 
and other properties of the water, the temperature fell 
rapidly to between 23° and 24° C., and to 22° C. in six 
fathoms off Flores Island close to Monte Video. The 
minimum temperature observed in this part was 20° C, 
at 2 a.m. between Lobos Island and Maldonado Point. 
Excluding the latter part of the voyage between the 
River Plate and lat. 15° S., where the conditions are a 
good deal affected by purely local causes, the surface- 
water shows well-marked diurnal maxima and minima of 
temperature. From lat.9° N. to lat. 2° N. the ship passed 
through the equatorial belt of calms and rains, which 
separates the regions of the north-east and the south-east 
trade-winds from each other. It is characterised by a 
calm sea, a cloudy sky, and heavy rains. Here the tem- 
perature was subject to very little diurnal variation 
(03 C.) On approaching St. Paul’s Rocks, a few miles 
north of the equator, the clouds cleared away completely, 
and there was a calm sea, a clear sky, and a very power- 
ful sun. The result was a comparatively great rise of 
temperature in the afternoon ; and yet the greatest differ- 
* Having plenty of pounded ice at my disposal, I poured off the water 
which had formed by melting, and replaced it by sea-water, containing 35 65 
grammes salt per kilogramme, and then immersed the thermometer ; it fell 
rapidly below zero, and remained constant at —1°‘o. I then strained away 
the sea water from the ice and replaced it by a mixture of equal volumes 
sea-water and distilled water : the thermometer fell to — 0°45, and remained 
constant for some time at that temperature. When the ice was mixed with 
distilled water alone, the thermometer again stood ato’ C. These experi- 
ments were made to verify some observations of Pettersson, quoted in his 
investigations into the nature of ice formed from waters of different degrees 
of salinity, in connection with the voyage of the Vega. He there says, re- 
ferring to the melting temperature of different kinds of ice, that pure fresh- 
water ice, when immersed in sea water, melts at a temperature considerably 
below o° C. Writing from memory, I think he puts the melting-point at 
from —1° to —2° C. _ Having both the ice and the sea-water ready at hand, 
I repeated this iemarkable experiment. ‘The result showed that Pettersson’s 
observation is quite correct, and that the lowering of the melting-point is 
roughly proportional to the salt held in solution. When equal volumes of 
the sea-water and distilled water of the same temperature were mixed, there 
was no change of temperature. I do not remember if Dr. Pettersson 
furnished an explanation of this remarkable phenomenon, and I am unable 
to supply one myself, but it must necessarily affect the validity of conclu- 
sions as to the composition of sea-water ice drawn frem its melting-point. 
When the Chadlenger was in Antarctic waters I made a number of observa- 
tions on the melting-point of ice collected from broken pieces of the pack, 
and found it begin to melt a little below —1°C. I concluded that either it 
Was one solid substance or a mixture of several solids. Butfif pure ice melts 
at a different temperature according to the medium in which it is placed, 
then this reasoning is faulty. for inclosed brine would have much the same 
effect as inclosed salt or crystalline hydrate. 
NATURE 
127 
ence between any neighbouring maximum and minimum 
in this region was only 1°1°C. 
The maximum temperature of the sea-surface observed 
during the voyage was 27°4C. (81°3 F.) at 2 p.m. on 
January 31, in lat. 7° 35’S., the Brazilian coast being 
about 100 miles distant. The temperature of the water 
will be further considered in connection with its density ; 
at present its connection with the temperature of the air 
will be more particularly considered. 
Temperature of the Air —Along w:-h the temperature 
of the water, that of the air during c.ylight was deter- 
mined. It is probably very rare, in any part of the 
ocean, to find the mean temperature of the air agreeing 
accurately with that of the surface water, and in many 
places the differences are considerable. In order to be 
able to compare the temperature of the air with that of 
the water, it is necessary that both should be determined 
with equal accuracy. The temperature of the water is 
easily and accurately determined by agitating the thermo- 
meter in a bucket of it freshly collected. With the air it is 
somewhat different. Having only one thermometer with 
me, I was obliged to use it for all purposes, and I could 
not hang it up in a thermometer-box, even if I had had 
one, and had deemed it advisable to do so. On board 
ship, however, I am convinced that it is quite impossible 
to fix a thermometer-box in such a position as always to 
secure such an air-pressure as to justify the assumption 
that the indications of the thermometer may be taken as 
the true temperature of the air. Even on shore and 
under the most advantageous circumstances, the tem- 
perature of the thermometer in the atmosphere of the 
best constructed box is too much dependent on the tem- 
perature and capacity for heat of the material of the box 
for it to be assumed always to be identical with that of 
the air outside, at the moment of reading. I was 
obliged, therefore, to adopt the method of whirling the 
thermometer, at the end of a short string, in the air, in 
whatever part of the ship happened at the moment to 
afford the most favourable conditions, and reading it 
when it had assumed a constant temperature. The tem- 
perature of the air is thus determined in mostly the same 
way as that of the water, namely, by agitating a thermo- 
meter in it, and the comparison of the two is therefore 
likely to lead to trustworthy conclusions. , 
Temperature of Wet-bulb Thermometer.—The series ot 
observations with this instrument is not so complete as 
that with the dry thermometer, but they possess some 
interest. The method of observation was the following :— 
The temperature of the air having been determined by 
whirling the thermometer in it, a bucket of sea-water was 
fetched and its temperature taken ; the thermometer was 
then exposed, with its bulb still wet with sea-water, to the 
breeze in a proper part of the ship, and its temperature 
observed when it became constant. The exposure of the 
instrument requires some care. The bulb must be quite 
free from grease, which can be readily secured by washing 
it with soap and water. It is then dipped into the water 
and allowed to drip for a second. It is then held some- 
what inclined to the direction of the wind and to the 
horizon, and rotated gently on its axis so that the bulb be 
kept covered with a continuous film of water which is 
locally thickened by gravity, which tends to form a drop 
on the lower side of the bulb. The reading of the thermo- 
meter is observed while it is being rotated. Had I in- 
tended from the beginning to make a series of wet and 
dry bulb observations, I should probably have used ‘fresh 
water from the first. I began to expose the thermometer, 
merely in order to have an indication whether the atmo- 
sphere were saturated or not, and I expected, in the damp 
equatorial regions, to find the atmosphere so heavily 
saturated as to be incapable of producing any sensible 
lowering of the thermometer with damped bulb. For 
this purpose it seemed to be quite sufficient to expose the 
thermometer wet with sea-water. Having begun with 
