
Fuly 13, 1871] 
blindly enraptured with that theory, as he is pleased to state, I 
only reply that very clever men have held it, as he is perhaps 
aware; and certainly none of the modern theories, cumbrous 
vagaries of the brain, can compare with it. I have never said 
that the meteoric theory is the real explanation, but I doubt if we 
shall ever arrive at a more truthful representation of the solar 
phenomena. 
Lastly, he culminates by saying I am ‘‘either innocently or 
wilfully ignorant of the palpably cyclonic appearance which spots 
frequently present.” All I can say in answer to this is that, 
having observed sun-spots myself for many years, probably as 
often as the reviewer, I have zever observed one single appear- 
ance of a cyclonic nature. As I possess Mr. Carrington’s valuable 
work, I have again referred to it, and find it in agreement with 
the assertion in my book and my own observation. I must 
apologise for my lengthy letter. A. M, Davis 
2, Gloucester Terrace, Sandgate, July 4 

On an Error in Regnault’s Calculation of the Heat Con- 
verted into Work in the Steam Engine 
In Watts’s ‘‘Dictionary of Chemistry” (vol. iii. p. 125), in 
the article on Heat by Prof. G. C. Foster, it appears to me that 
an important error has crept into the discussion of the above cal- 
culation, 
The nature of the calculation is as follows :—A unit weight of 
saturated steam at the temperature of 152°C. contains 653 units of 
heat. Suppose we allow the steam to expand and to do work 
until the temperature falls to 503°C. the steam then contains 621 
units or 32 units lessthan before, hence starting with water at o°C., 
we give it 653 units of heat, and of this 32 only are converted 
into work, giving us the fraction 82; as the amount of heat con- 
verted into work ; but the real work produced by an engine is 
more than twice this. This difference in theory and practice is 
accounted for by the fact that saturated steam, in expanding and 
doing work, is partly condensed, hence the body with which we 
have to deal at the lower temperature is not all steam, but partly 
condensed water, therefore, does not contain so much heat as was 
allowed it. 
This explanation is so intelligible as to be at first sight suffi- 
cient to account for the whole difference ; there is, however, 
another cause, quite as important, and which is this ; every time 
steam passes from the boiler to the cylinder it does work before 
it is cut off, and allowed to expand ; this work is not done at the 
expense of the steam that passes into the cylinder, but of the 
whole mass of steam in the cylinder and boiler, which expands 
and is thereby cooled. The mass of water and steam in the 
boiler is, however, so large compared to that which passes into 
the cylinder, that a thermometer could scarcely detect the cooling 
effect upon it, and before the next stroke this loss of temperature 
is made up by the fire. Though thus inappreciable, it is never- 
theless very important, and in most engines would amount to 
one-third the work done ; in fact all the work done by the steam 
before it is cut off and allowed to expand is entirely neglected in 
this calculation, and a source of error introduced. 
To correct it there should be added to the heat in the steam at 
the initial temperature, as many units of heat as the work done 
before the steam is cut off, would, if converted into heat, raise 
the amount of water which passes at every stroke in the form of 
steam into the cylinder. A. W. BICKERTON 
Hartley Institution, Southampton, June 26 


THE CAUSES OF THE COLOURS OF THE SEA * 
ROF. TYNDALL, in his article in the Fortnightly 
Review for the 1st of March, attributes the green- 
ness of the sea to the matter which it holds in solution. 
Perhaps the following may corroborate his theory. About 
the Andaman Islands, where the sea is of the deepest 
blue, there are most startling and sharply-defined changes 
of colour, from bright blue to green, where a bed of coral 
exists. This coral is white out of the water, what its 
colour when growing may be I know not, but the change 
I mention appears to corroborate the remarks in the 
article in question, which are appended below, about the 
green hues observed upon the plate, the screw blades, and 
* Communicated by Prof, Tyndall. 

NATURE 203 

the white bellies of the porpoises. One looks down from 
a hill into a bay of the brightest blue ; you see it broken 
up here and there like a child’s puzzle map by irregular 
patches of as bright green, often crossing several acres as 
sharply defined as it is possible to imagine, and indicating 
the existence of coral beds or reefs just below. 
Bellary, Madras Presidency W. M‘MASTER 
[We give the passages referred to from Prof. Tyndall’s 
lecture.—ED. | 
“*Let us clear our way by a tew experiments towards an ex- 
planation of the dark hue of the deep ocean.* Colour, you 
know, resides in white light, appearing generally when any con- 
stituent of the white light is withdrawn. Here is a liquid which 
colours a beam sent through it purple, and this colour is im- 
mediately accounted for by the action of the solution on a spec- 
trum. It cuts out the yellow and green, and allows red and blue 
to pass through. The blending of these two colours produces 
the purple. Does the liquid allow absolutely free passage to the 
redand blue? No. It enfeebles the whole spectrum, but attacks 
with special energy the yellow and green colours. By increasing 
the thickness of the stratum traversed by the beam, we cut off 
the whole of the spectrum. Through the deeper layer, which I 
now place in the path of the beam, no colour can pass. Here, 
again, is a blue liquid. Why is it blue? Its action on the 
spectrum answers the question. It first extinguishes the red ; 
then as the thickness augments it attacks the orange, yellow, and 
green in succession ; the blue alone finally remains, but every- 
thing might be extinguished by a sufficient depth of the liquid. 
“And now we are prepared for a concentrated but tolerably com- 
plete statement of the action of sea water upon light, to which it 
owes its blackness, Here is our spectrum. This embraces three 
classes of rays—the thermal, the visual, and the chemical. These 
divisions overlap each other ; the thermal rays are in part visual, 
the visual rays in part chemical, and vice versd. The vast body 
of thermal rays is here beyond the red and invisible. They are 
attacked with exceeding energy by water. They are absorbed 
close to the surface of the sea, and are the great agents in evapo- 
ration, At the same time the whole spectrum suffers enfeeble- 
ment ; water attacks all its rays, but with different degrees of 
energy. Of the visual rays the red are attacked first, and first 
extinguished. While the red is extinguishe:l, the remaining 
colours are enfeebled. As the solar beam plunges deeper into 
the sea, orange follows red, yellow follows orange, green follows 
yellow, and the various shades of blue, where the water is deep 
enough, follow green. Absolute extinction of the solar beam 
would be the consequence if the water were deep and uniform, 
and contained no suspended matter. Such water would be as 
black asink. A reflected glimmer of ordinary light would reach 
us from its surface, as it would from the surface of actual ink ; 
but no light, hence no colour, would reach us from the body of 
the water. In very clear and very deep sea water this condition 
is approximately fulfilled, and hence the extraordinary darkness 
of such water. The indigo, to which I have already referred, is, 
I believe, to be ascribed in part to the suspended matter, which 
is never absent, even in the purest natural water, and in part to 
the slight reflection of the light from the limiting surfaces of strata 
of different densities. A modicum of light is thus thrown back 
to the eye, before the depth necessary to absolute extinction has 
been attained. An effect precisely similar occurs under the mo- 
raines of the Swiss glaciers. The ice here is exceptionally com- 
pact, and owing to the absence of the internal scattering common 
in bubbled ice, the light plunges into the mass, is extinguished, 
and the perfectly clear ice presents an appearance of pitchy 
blackness, 
“The green colour of the sea when it contains matter in a state 
of mechanical suspension has now to be accounted for ; and here, 
again, let us fall back upon the sure basis of experiment. This 
white plate was once a complete dinner-plate, very thick and 
strong. It is, you see, surrounded securely by cord, and to it a 
lead weight is fastened. Forty or fifty yards of strong hempen 
line were attached to the plate. Withit in his hand, my assistant, 
Thorogood, occupied a boat fastened as usual to the davits of 
* A note written to me the 22nd of October, by my friend Canon Kingsley, 
contains the following reference to this point :—‘‘I have never seen the Lake 
of Geneva, but I thought of the brilliant, dazzling dark blue of the mid At- 
lantic under the sunlight, and its black blue under-cloud, both so solid that 
one might leap off the sponson on to it without fear ; this was to me the most 
worn thing which | saw on my voyage to and from the West Indies.”— 
‘ 
