Feb, 2, 1871] 
NATURE 
265 
eee 
In Nature of Jan. 26, Mr. Munro calls attention to the great 
brilliancy and saturation of many natural yellows as accounting 
for the difficulty of resolving them into their components. _ It is, 
no doubt, quite true that a full yellow could not be compounded 
of such reds and greens as we come across in daily life, but it is 
equally certain that a drab or dilute yellow could be ; and yet no 
one recognises the fact by his unaided senses, or thinks it any- 
thing but strange and unlikely when told of it. And afier all, 
can it properly be said that natural yellows are more saturated 
than other colours? That they approach more nearly the cor- 
responding tints in the spectrum is admitted ; but is that test a 
fair one? It seems to me that the homozeneous yellow itself 
must be considered as dilute when brought into comparison with 
the nearly primary red and green. 
I have another difficulty inaccepting Mr. Munro’s explanation. 
A suitable mixture of any red, green, or blae will give a neutral 
grey. All four come within our every-day experience; but such 
a result seemed to Goethe, soon after Newton proved it, a para- 
dox of paradoxes, and I believe to unsophisticated minds it seems 
so still. 
Mr. Munro has ingeniously shown from the colour equations 
that there is no more primary blue in my blue dise than about 
24 as much as in the red f/ws 14 as much as in the green—a con- 
clusion which seems somewhat startling. In choosing the coloured 
papers and cards for the discs, I had great difficulty in finding a 
green that was even tolerably good, and the one that I finally 
used reflected large quantities of blue light. I had some thought 
of trying a green silk disc, whieh was of a much better colour, 
but feared errors depending on the different character of the 
surface. 
It is not hard to see a reason for the comparative scarcity of 
good greens. To obiain a good red orange or yellow by means 
of absorption, all that is necessary is to cut away the spectrum 
above a certain point; for a good blue, the rays standing 
below a given one in refrangibility must be got rid of ; but in 
order to isolate a green in anything like putity, the absorbing 
agent must hit off ¢wo points of the spectrum, removing all 
below one point and all above the other. The result is, that 
while nearly saturated yellows and reds abound—the scarlet 
of the geranium is almost perfect—hardly a good green is to be 
met with. The best I know is a mixture, prepared by 
adding bichromate of potash to a_ strong solution of sulphate of 
copper. The addition of a little chloride of chromium to remove 
the yellow more effectually is perhaps an improvement. If Mr. 
Munro would care to see the colours which I used for the discs, I 
should be very happy to send him samples. 
Terling Place, Witham, Jan. 29 J. W. STRUTT 
Comets’ Tails 
May there not be a connection between the colour of the sky 
and the tails of comets ? ‘ ; 
Suppose a comet to be surrounded with a wide-spread trans- 
parent atmosphere, holding in suspension matter as finely di- 
vided and as invisible as the sky-matter of our earth ; and suppose 
the more condensed, but still transparent nucleus to act as a lens, 
throwing a beam of light upon the otherwise invisible mist of 
that atmosphere, could not most of the phenomena of those 
puzzling tails be thus accounted for ? 
Leicester, Jan. 28 FREDERICK T, Morr 
Ocean Currents 
Mr. Laucuron’s letter in the latest number of NATURE 
shows that the suggestion of a probable influence of differences of 
atmospheric pressure on ocean currents has not been stated with 
sufficient clearness. 
That a hydrostatic equilibrium would exist under a permanently 
unequal distribution of the pressures over the ocean, as Mr. 
Laughton argues, is not to be doubted. But is there any such 
permanent arrangement ? d ; 
Granting that the high-pressure area of the trade-winds is 
nearly constant, varying little from day to day, and only expand- 
ing and contracting its limits gradually in summer and winter, 
the pressures to northward of this over the Atlantic are by no 
means so steady. Though they give on the average of the year, 
or even of the month, a /ower pressure, yet the daily observations 
show that the pressures over wide areas may vary considerably, 
rising at one time to equal that of the Trade-wind patch, at 
another falling very much below it. Quoting from the daily 

barometric curves registered in the Quarterly Report of the 
Meteorological Office (for April—June, 1869), **On the 13th of 
May in that year, the seven stations whose weather is there re- 
corded (Valencia, Armagh, Glasgow, Aberdeen, Falmouth, 
Stonyhurst, and Kew) show simultaneously a barometric pressure 
of 30°1 inches.” On the 6th of May these stations had severally 
reported a pressure of only 28'9 inches. A difference of pressure 
| equal to that of 16 inches of water had taken place over the 
entire extent of the British Isles, and probably over a much 
greater space, 
The system of synchronous observation is not yet sufficiently 
| extended to enable any positive statement of the general direction 
of the movement of low pressure areas across the North Atlantic 
to be made ; but there is a strong probability that such depres- 
sions travel in a direction continuing the path taken by the hur- 
ricanes of tiie West Indies, since the majority of barometric 
hollows reach the British Isles from west or south-west. Just 
as the waters are forced to rise into the central low pressure of a 
hurricane (to the extent of several feet) and to follow its path, so, 
in a less degree, does it seem probable that the movement of ex. 
tended and less violent dep.essions may influence the ocean 
currents. 
The rate of progress of such depressions appears to be in an 
inverse ratio to their extent and depth. A West Indian hurricane 
moves onward at a rate of only fifteen miles an hour (Buchan’s 
Meteorology, p. 269), and the highest speed of European storms, 
according to the same authority, is forty-five miles an hour; but 
that minor depressions may travel with much greater velocity is 
shown by reference to the quarterly report before noticed, where 
is the record of a hollow of from three to four tenths of an inch, 
which passed over the British Isles on the 24th of February, 
1869, at the rate of ninety miles an hour.* 
Either in causing a considerable change in the level, or in 
rapidly moving over it, from near an area of constantly high pres- 
sure, through a region where the average pressure diminishes, 
such depressions must surely influence the surface of the ocean, 
and either aid orretard its currents. That a difference in level of four 
inches in 1,800 miles can scarcely under any circumstances give 
rise to a current of twenty miles a day (the word hour in Mr. 
Laughton’s letter is probably a typographical error) is also clear, 
but the temporary difference in level may be much greater than 
this within a much shorter distance. The average rate of the 
north equatorial current, moreover, in the Trade-wind region, is 
shown by the pilot charts of the Admiralty to be only from ten 
to twelve miles a day, and it is only claimed for difference of at- 
mospheric pressure that it has some small share in aiding the 
formation of the current in question. 
KEITH JOHNSTON, JUN. 
Insulation of St. Michael’s Mount, Cornwall— 
When did it occur ? 
THis is a very interesting question, and the reader will be 
enabled to judge for himself, presently, whether there are not 
sufficient historical facts on record to enable us to answer—‘‘ In 
the eleventh century.” 
Domesday Book, date 1086, in the part relating to ‘‘ Corn- 
valge” (¢.e. Cornwall), at p. 2, has the following, which I have 
translated from the abbreviated Latin—‘t Ze Land of St. Michael. 
—Keiwal holds the church of St. Michael. rismar was holding 
it in the reign of King Edward. ‘There are two hides which 
never paid the Danish tax [nunquam geldaverunt]. ‘Ihe land is 
S carucates. There is 1 carucate with 1 vilian, and two bordarii, 
and 10 acres of pasture. Value 20 shillings. Of these 2 hides, 
Earl Moriton took away 1 hide, value 20 shillings.” And ac- 
cordingly, at p. 11 of Domesday Book, there appear in the 
descriptive list of the many estates of Earl Moriton, corresponding 
particulars of the 1 hide which he had taken away. 
Now, in the first place, Domesday Book gives no reason what- 
ever for believing that, at its date, St. Michael’s Mount was an 
4sand, neither does Magna Britannia, vol. 1, p. 309, where the 
Mount is called AZychel-stop, or Michael’s place. And in every 
case, while *‘annoting” those holding possessions in ‘‘ Corn- 
valge,” there is an entire absence in Domesday Book of any 
meation of island or islands on any of the coasts of Cornwall, 
justas if there had been then no islands on those coasts of sufficient 
extent to be worthy of mention. On the other hand, it is the 
* The Meteorological Report shows that in January, 1769, 12 d=pressions 
reached the British Isles from W.-ward ; in February 10, also from W.; in 
March 8 (5 from W.) ; in April 7 (5 from W.) ; in May 8(7 from W.); and in 
June 2 from W. 
