390 
INCI OI as 
[FEBRUARY 25, 1897 
that we could make a machine to utilise some of the energy in 
the ether ; but does any one profess to know so much of that re- 
markable thing as to be quite certain that this isimpossible ? Any 
way, there is no doubt that to a considerable degree of accuracy 
inertia zs a constant property of a body, and equal inertias may 
consequently be very reasonably considered as equality of such a 
very important property of two bodies, that scientific people are 
justified in their shortly describing the bodies as equal, which 
1s what they usually do, and is all that they can really mean 
when they speak of equal quantities of iron and gold. Why, 
then. trouble unfortunate students with the idea that there is 
some huggermugger metaphysical ‘* quantity of matter” called 
“mass,” of which they are supposed to have a clear and definite 
conception distinct from this equality of inertia? Why not call 
it inertia when it is inertia that is meant, and drop out of use 
that word ‘* mass,”’ round which such a tissue of indistinct and 
obscure ideas have grown, that it is almost hopeless to separate 
it from them, 
I hope some word more euphonious than ‘‘slug” will be 
found for the unit of inertia on the engineer’s system. I would 
suggest “ert” as a term that would easily recall the quantity 
inertia, GEO. FRAS. FITZGERALD, 
Trinity College, Dublin, February 10. 
The Flight of Gulls in the Wake of Steamers, 
MANY persons have remarked the extraordinary power dis- 
played by gulls of keeping pace with a steamer without any 
motion of their wings. A few days ago, I had a good oppor- 
tunity of observing this during a voyage from Alexandria to 
Marseilles. 
When the wind was blowing at right angles to the course of the 
vessel, having first gained some slight elevation, the gulls would 
glide downwards with expanded wings, making, during the de- 
scent, rapid progress in the same direction as the steamer. When 
quite near the water they would suddenly turn and face the 
wind, at the same time giving their bodies an upward incline, 
and the wind would lift them to their former elevation, after 
which the process would begin again. A wind blowing hori- 
zontally has the power of lifting, only because each stratum, so 
to speak, of air moves more rapidly than the stratum immediately 
below it. Consequently, as the bird rises, it has the inertia 
due to the fact that it has just emerged from the slower current 
below. Thus it may be compared to a kite, the inertia taking 
the place of the string. When gulls progress in this way, at 
right angles to the wind, the vessel does not in any way assist 
them, and, occasionally, when they are not following a steamer, 
they may be seen employing the same method. 
With a head-wind they advance with even greater ease. To 
understand how this is possible, some investigation of the air- 
currents behind the ship’s stern is necessary. If small pieces 
of paper are thrown overboard when a strong head-wind is 
blowing, they are seized by a tremendous down-draught, but, 
some few yards astern, they suddenly dart up again. In fact, 
as the vessel moves onward, the air rushes down to fill the 
vacuum, then rebounds off the surface of the sea, and forms an 
up-current. Placing himself in this up-current, the gull is 
lifted as if he were no heavier than a scrap of paper, then he 
glides downward and onward. But as the vessel moves on, the 
up-current advances, or, strictly speaking, the point at which 
the up-current is formed. At the end of his descent the gull 
finds himself in this, is again lifted, and the process is repeated. 
When the wind was not a due head-wind, but struck the 
vessel at a slight angle, now and then a gull would be seen 
apparently hovering motionless over the stern, of course really 
gliding onward with the vessel. Though I cannot speak with 
confidence of the explanation of this, the most wonderful of the 
methods employed, I wish to put forward what seems the 
probable explanation. The wind striking against the side of 
the vessel is deflected upwards, and it is this up-current which 
buoys up the gull as he floats over the stern. Though it may 
appear that his progress is perfectly uniform, I think it will be 
found that in advancing he descends slightly, that he often loses 
ground for a time, and that while losing ground he ascends. 
Thus the method in this case is really the same as in that last 
described. Unfortunately, I was not able to prove the existence 
of this up-current about 20 feet above the stern of the vessel. 
But there is good evidence of it in the fact that the gull remains 
suspended there without a motion of his wings. Without an 
NO. 1426, VOL. § ] 
up-current this would be an impossibility. It is to be hoped 
that good observers will give their attention to these very 
interesting phenomena. F. W. HEADLEY. 
Haileybury, February 8. 
Two Unfelt Earthquakes. 
On February 7, commencing at about 8 a.m., G.M.T., an 
unusually large, but, at the same time, unfelt earthquake was re- 
corded in the Isle of Wight. The preliminary tremors, which 
include three well-defined maxima, extended over twenty-six 
minutes. After these came two periods of heavy movement, 
each extending over fifteen or twenty minutes. The duration of 
the whole disturbance was about one and one-half hours. It was 
Japanese in character, and because it was recorded in Tochia by 
Dr. G. Grablovitz, and at the same time was so marked in ampli- 
tude and duration, it is not unlikely that it disturbed the entire 
surface of the globe. 
On the 13th there was a comparatively small disturbance, with 
preliminary tremors of three or four minutes, at about ro a.m. 
I should be pleased to learn whether these earthquakes were re- 
corded by bifilar pendulums in Edinburgh or Lirmingham, or 
at any of our magnetic observatories. JOHN MILNE. 
Shide, Newport, I.W., February 18. 
FOUNDATIONS OF CORAL ATOLLS. 
HE most regrettable failure of the boring lately 
attempted in the coral atoll of Funafuti has left us as 
wise as we were as to the actual structure of these forma- 
tions ; but the surveys carried on by H.M.S. Penguzn, both 
at Funafuti and in the regions round about, have afforded 
information which, I think, is of value in elucidating 
some of the problems to be solved, and which has 
certainly strengthened some of my own views on the 
subject. 
Funafuti, it may be mentioned, was selected for in- 
vestigation as being one of a great Pacific group of atolls, 
which must have a common great cause for the forma- 
tion of their necessary foundations, and for their develop- 
ment ; groups which had a great share in causing Mr. 
Darwin to conclude, from the lack of other explanation of 
banks in large numbers at a proper depth for the growth 
of an atoll, that subsidence on a large scale was the 
predominant agent in their production (‘‘ Coral Reefs,” 
2nd ed., pp. 118, 119 ; 3rd ed., pp. 120, 121.) 
Firstly, the sounding carried on by the Penguin round 
Funafuti and between separate islands of the Ellice 
Group, show incontestably that each atoll is situated on 
a separate mound, rising from a more or less even 
bottom of great depth below the surface. This proves 
that there has never been anything of the nature of a 
range of continental land which has gradually sunk 
beneath the waves. Each atoll, if it has sunk, has sub- 
sided independently, with its own isolated volcanic peak. 
Secondly, the Penguin, while searching the seas some 
250 miles to the south-westward of the Ellice Islands for 
several reported dangers to navigation, explored four 
banks, all of submerged atoll form, lying near one 
another. 
The remarkable thing about these banks is the abso- 
lute uniformity of the depth of water over their areas, 
inside the low rim of growing coral which encircles 
their edges in various degree. This depth is 24 to 26 
fathoms. The banks are large: one is 22 miles by 10; 
another is 18 miles by 9; the third is 8 by 7; and the 
fourth 4 by 3. The plan of one of them is given on the 
next page as an example. 
Another bank, investigated a few years ago by H.M.S. 
Waterwitch, and lying 400 miles to the eastward, 
presents similar characteristics, and the same depth over 
its central area. All these banks are situated in a region 
exposed to the same conditions of wind and sea. 
What causes this remarkable similarity of depth and this 
extraordinarily even surface over these large banks? Is 
