346 
NATURE 
[March 2, 1871 

II. (1.) Localise your pound of force by specifying the stan- 
dard locality of reference where the imperial pound gravitates 
with unit force. For this unit to be generally adopted, there 
must be general agreement as to the locality of reference, or 
what amounts nearly to the same thing, general agreement as to 
a definite reference-value of g. If we make this 32°2, the unit 
of force, at a locality where g has any other value, will be Bee 
& 
of the local gravitating force of the imperial pound, and the unit 
of mass will be the mass of 32°2 imperial pounds, so that the 
units will be a definite multiple of those employed in I. 
This system is sound because (though in a roundabout way) it 
acknowledges the pound as a universal standard of mass, which 
is everywhere to be denoted by the same number, while it de- 
notes the gravitating forces of pounds in different latitudes by 
different numbers. 
If we take as unit of force the gravitating force of half an 
ounce at a place where g is 32, we obtain a clumsy definition of 
the Gaussian unit of force, and our unit of mass becomes the 
pound. 
(2.) File or load your pounds so as to make a pound at the 
pole gravitate with the same force as another and larger pound 
at the equator. This is the only way of making the pound a 
direct standard of force to the inhabitants of the world generally. 
The unit of mass will then be g times the mass of one of these 
filed or loaded pounds, and will everywhere represent the same 
mass. 
(3.) Let every man adopt the local gravitating force of an im- 
perial pound at the place where he happens to be, as his unit of 
force, and the mass of g imperial pounds as his unit of mass. 
This system gives a rough and ready unit of force, which is fre- 
quently adopted for rough purposes by all physicists ; but expe- 
rimental results stated in terms of it must be accompanied by a 
statement of the local value of g, to make them comparable with 
those obtained at other places. In dealing with masses in cases 
where forces are merely subsidiary, as in buying and selling goods, 
no one would recommend the adoption of the unit of mass which 
this system gives. In cases where forces and masses have to be 
considered in conjunction, this system has no advantage over I. 
in point of simplicity, and has the disadvantage of requiring us 
to express both forces and masses in terms of changeable units 
which tend to confusion of ideas. It is altogether inapplicable to 
astronomy, and is not even competent to express the mass of the 
earth ; for it would make the mass of a cubic foot of matter at 
the earth’s centre many millions of times greater than the mass 
of a cubic foot of gold or platinum at the earth’s surface. 
Writers who, without special explanation, express forces in 
pounds or grammes, and say that the mass of a body is numerically 
equal to its weight divided by g, must be classed as adopting this 
system; for though such expressions are ambiguous in themselves, 
this is the sense in which they will usually be received and applied. 
It is indeed the system which was almost universally taught until 
the publication of Thomson and Tait’s Natural Philosophy. 
I am not quite clear as to the particular system which 
‘““W.M. W.” elects toadopt. He began (p. 145) by siding with 
Deschanel, who seems to adopt II. (3). In answer to my first 
letter (p. 167), he stated (p. 187) that ‘‘if a true pound, as 
determined at London, were carried to the North Pole, it would 
weigh more than a pound.” If this be not an adoption of II. (2), 
itamounts to saying that at the North Pole a pound does not 
weigh a pound. In the second sentence of his last letter he 
alopts II. (1) without, however, distinctly committing himself to 
a definite locality of reference ; and as long as this point is left 
open, every man will make the locality where he happens to be 
the locality of reference, so that II. (1) in this indefinite form de- 
generates into IT. (3). In the same letter he says, ‘‘ As a philo- 
sophical theory, I am perfectly ready to admit that the standard 
pound is most appropriately considered as a standard of mass, 
but the employment of this standard in a text-book for the use 
of beginners seems calculated to lead to confusion.” It rather 
appears to me that the refusal to accept this real standard of 
mass leads usually to a confused mixture of the systems II. (1), II. 
(2), II. (3); and I have shown in this letter that II. (1), which is 
the best of the three, does, in fact, make the pound a standard 
of mass. 
I would earnestly commend to all teachers of dynamics the 
practice of Sir W. Thomson in strictly abstaining from the use of 
the word weig/¢ in all definitions and specifications relating to 
mass and force, as its ambiguous use does more than anything else 
to-confuse these subjects. The weight of a body may and most 

frequently does mean its mass stated in pounds, or it may mean 
the force with which it gravitates. In the one sense its weight is 
the same whatever place it is carried to; in the other sense it 
varies from place to place. - 
I would also recommend for imitation Sir W. Thomson's 
practice of discarding the term accelerating force, which has been 
used to denote what ought to be called force per unit of mass or 
intensity of force, as distinguished from amount of force. Forces 
should always be expressed in terms of comparable units. As 
long as the learned recognise two non-comparable measures of 
force, they can hardly blame the unlearned for recognising a third 
and confounding force with energy. 
The phrase absolute force (of a centre) is for the same reason 
objectionable. Strength of a centre is a better designation, and 
magneticians already speak in this sense of the strength of a 
magnetic pole. It would be a great advantage to have a short 
and handy name for some unit of force properly so called, and I 
venture to propose that the unit of force defined in I. be called 
a kinit. It may be formally defined as that force which, acting on 
an avoirdupots pound of matter for a second, generates a velocity of 
a foot per second. If we substitute gramme for pound, and metre 
for foot, we obtain a different unit which must be called by a 
different name, and of which 1384 make one kinit. This 
numerical relation remains true, even if one of the forces com- 
pared be at the earth and the other at the moon. If any etymo- 
logist objects to making a word derived from Greek end in #, he 
may adopt the convenient fiction that it is an abbreviation of 
kinetic unit, J. D. EVERETT 
Belfast 
The Spectrum of the Aurora 
In the Philosophical Magazine for February there is a 
paper by F. Zéllner, on the Aurora Spectrum, in which he 
points out that, since the light of the aurora is as faint as that 
of the faintest vacuum-tube capable of being spectroscopically 
examined, while the mass of incandescent gas is almost infinitely 
greater, its temperature must be exceedingly low comparatively. 
He therefore supposes “that the spectrum of the aurora borealis 
does not correspond with any known spectrum of the atmospheric 
gases, only because, though a spectrum of our atmosphere, it is 
one of another order, and one which we cannot yet produce 
artificially.” 
I do not know if any other observer has noted the apparent 
coincidence in position of several of the auroral bands with those 
of a spectrum which I have occasionally obtained from air at low 
pressure, and with a feeble discharge. It is sometimes exhibited 
withgreat brilliancy by ordinary “lumiére” tubes, and is, I 
believe, in part at least, the spectrum described by Wiillner 
(Phil. Mag., June 1869), as a new spectrum of oxygen. 1 have 
certainly obtained it very vividly in pure electrolysed oxygen, 
with a feeble discharge ; but some perplexing observations make 
me rather doubtful of its origin. 
In the annexed sketch No, I represents the spectrum above 


Ha Na HB ~ Ily 
mentioned, No. 2 that of the aurora, while No. 3 gives the lines 
of H and Na for comparison. aj 
I first noticed the coincidence of the yellowish-green line-at 
41 with the principal line of the aurora in Jan, 1870, and since 
that time have frequently repeated the observation ; once with a 
spectroscope with a 60° bisulphide prism, and magnifying power 
of about six times. With this instrument the lines of both spectra 
appeared nebulous, but perfectly coincident. When the light 
from the vacuum tube is strong, this band appears shaded off 
gradually on the most refrangible side, but when it is no brighter 
than the aurora this is not noticeable. 
The faint lines shown in the sketch were never bright enough 
to be compared with the same accuracy as the bright line, but 
repeated observations convince me that the positions given can- 
not be far wrong. Their relative brightness is very variable, 
