The Sun.
London, England, 1860-07-25 00:00:00

TO THE EDITOR OF THE SUN.

Stit,—A total eclipse of the sun is a phenomenon
which attracts universal attention, and being of
rare occurrence, and more especially so in any par-
ticular spot, when such an eclipse does take place
an excitement is caused so great that it takes pre-
cedence in the minds of men over everything else,
for that day at least. It will therefore, doubtless,
interest your readers to hear something regarding
the great eclipse of the 18th of July.
Before describing this particular eclipse, allow
ni.e to say a few words on eclipses in general. In
the present age, unlike olden times (when a super-
stitious dread filled the minds of men on such occa-
sions), we all know more or less about eclipses ; we
have learnt that a solar eclipse is nothing more nor
less than the body of the moon passing between us
and the sun, and that a lunar eclipse is caused by
the earth passing between the sun and the moon,
and thus preventing the sun shining upon it, or, in
other words, the earth casts a shadow upon the
moon, and this shadow is the darkness which we
see upon the moon in a lunar eclipse. Perhaps,
however, it is not universally known that the same
eclipses recur every 18 years and ten and a-half
days. These eclipses will differ in magnitude, and
they will occur more or less irregularly. Seventy
eclipses usually take place during
.the 18 years,
those of the sun being 41, and those of the moon
29. Reading this in an astronomical work, the
public naturally conclude that solar eclipses are
common, but on account of the moon's body being
smaller than that of the sun., her shadow is a cone,
tapering to a point, and as the distance between
the earth and moon slightly varies, it happens that
this point of the moon's conical shadow sometimes
does, and sometimes does not, reach the earth, and
when it does its dimensions are inconsiderable.
Consequently, a total eclipse is a rare phenomenon,
and, further, being limited in extent, it is even
more rare at any one locality. The immortal
Halley, in describing the total eclipse of the
sun in London of the 3d of May, 1715, remarked'
that previously none had been total in Lon-
don since March 20, 1140, a period of nearly 600
years. To take an example of the recurring pe-
riod of 18 years, by comparing the year 1842 with
the present year 1860, we have the following
eclipses recurring:—An annular solar eclipse, 1842,
January 11, and 1860, January 22 ; a partial lunar
eclipse, 1842, January 26, and 1860, February 6 ;
a total solar eclipse, 1842, July 7, and 1860, July
18 ; a partial lunar eclipse, 1842, July 21, and
1860, August 1; an annular solar eclipse, 1e42,
December 31, and 1861, January 10.
The period of totality of a solarseclipse can never
exceed 7m. 58s. under the equator, and not above
6m. in this island, so that even when we are fortu-
nate enough to witness a total eclipse of the sun,
its duration is necessarily very short, and conse-
quently phenomena of a wonderful character are
so crowded together that before there is time to
make a proper record daylight has returned. end
shut out these singular appearances. Indeed, the
sights to be seen during these brief minutes are of
so imposing a nature that it is only the steady
astronomer who can look and record what he sees,
for on no occasion is the motto of the Royal Astro.
nomical Society (" Quidquid nitit notqndum") more
difficult to be obeyed than during a total solar
eclipse.
Returning to the present eclipse, the line of
totality enters North America from the Pacific
Ocean at Astoria, in Oregon, passes across North
America to Cape Chudleigh, in Labrador, and across
the North Atlantic Ocean (the line lying to the
South of Greenland), enters Spain about Gijon,
Santander, and Bilbao, crosses Reynosa, Arnedo.
Calatayud, Daroca, Montalvin, Morella, and Oro-
pcsa, enters the Mediterranean, crosses the Island
of Ivica and grazes the south edge of Majorca,
passes into Algeria, and ends near the*Red Sea.
Tlie breadth of the line of totality is great ; the.
southern limit in Spain extends considerably
beyond Oviedo and Valencia, while the northern
limit extends beyond Tortosa and nearly 'reaches
Pampeluna.
Previous to describing the expedition and the ob-
servations made in Spain it will be necessary to
say a few words regarding the instruments. My
own department was meteorology, and the making
of the whole of the instruments was intrusted to
the care of Messrs. Negretti and Zambra, opticians.
Hatton-garden ; and I must add that my confidence
with regard to the skill of this well-known firm
was not misplaced. These instruments consist of a
standard barometer, patent maximum and minimum
thermometers placed at 1,2, 3, and 4 feet above the
ground, both in sunshine and shade, self-registering
maximum and minimum wet and dry bulb thermo-
meters, vacuum thermometers, thermometers made
expressly for plunging various depths into the
ground, and a series of blackened bulb thermome-
ters, a Robinson's anemometer to show the velocity
in miles of the movement of the air, and a Lowe's
ozone box; the wind vane being a light silk flag.
These instruments were read off, as previously ar-
ranged, every five minutes from 12 o'clock till 5
o'clock by myself and the Rev. W. R. Almond,
F.R.A.S , and Mr. Samuel Morley, my assistants,
accurate time being kept by a most excellent duplex
watch, made by Mr. J. Bennett, of Cheapside, a
maker whose watches I have used with satisfaction
for some years. The thermometers were placed on
a Lawson thermometer stand, made portable for
this expedition. For photographic observations,
salted papers (sensitized before leaving) were con-
veyed in the patent preservative case of M. Marion,
of Regent-street, and were exposed constantly dur-
ing the progress of the eclipse. Besides these, the
variously-prepared dry plates (i.e. collodio-albu-
men, metagelatine, Fothergill's, and those of the
Birmingham Dry Plate Company) were taken out
for exposure in the stereoscopic camera known as
Dancer's patent. These dry plates will be interesting
in more ways than one, as they will be the means
of recording pictorially the place of observation
and places of interest passed over by the present
expedition.
It will be necessary to add that the observers
were distributed in various directions. A large
portion which landed at Bilbao, incluiting the As-
tronomer Royal, have not as yet been heard of since
the eclipse; those observers who landed at Santan-
der took up positions—some at Santander, others at
Alar del ney ; Mr. Lassen. at Aguilar de Campo ;
the Swedish astronomers at Escudo Pass; Professor
Swann, Mr. Ellis, and myself, at Fuente del Mar;
Mr. Heath at Pena Castilla ; Mr, J. Mould at Los
Corrales ; and a staff of observers under Capt. Sec-
combe opposite the anchorage of the Himalaya. At
Fuente del Mar the beginning of the eclipse was
well seen. Exactly one minute before totality
clouds hid the sun, and the celestial phenomena
were lost ; but, owing to large spaces of clear sky,
the terrestrial phenomena were grandly visible,
and our whole attention was rivetted upon them.
Fortunately, a mile from here the observers at the
Himalaya had a fine view of totality, stars. became
visible, together with the red prominences and
corona.

The following is a record of the readings of the
different instruments, and, as they are taken from
above 4,000 observations, made by myself and as-
sistants during five hours, they must prove of great
value. Commencing with underground tempera-
ture, a thermometer placed 6 inches below the sur-
face of the ground ranged between 67.9 deg and
70.7 deg—i. e., 2.8 deg; at this depth the eclipse
was not sensibly felt, whereas other thermometers,
placed 4 inches, 2 inches, 1 inch, and half an inch
below the surface, all exhibited in a very marked
manner the effect of the eclipse. On the grass the
temperature fell to 64 deg at 3h 5m ; at half an inch
below the surface to 69 deg at 3h 15m ; at 1 inch
deep to 69.5 deg at 3k 25m ; at 2 inches to 71 deg
at 3h 55m ; and at 4 inches to 70.7 deg. at 4h
30m p.m.

The temperature on grass was 77.5 deg at noon,
rising to 91.7 at lh 50m, and then falling till
3h sm, and again rising to 85deg at 4h 10m, giving
a range of 27.7 de. At half an inch below the sur-
face of the ground the temperature rose till lh 53m
p.m., when it was 78.5 deg, and then gradually fell
to 69 deg at 3h 15m, rising again to 74.7 deg at
4h 30m p.m., the range being 9.5 deg. At 1 inch
below the surface the temperature rose till lh 55m
to 76.2 deg, fell till 3h 25m to 69.5 deg, and rose
till 4h 55m to 74.7 deg, the range being 6.7 deg ; at
2 inches below the surface the temperature rose till
2h sm—viz., to 74.4 deg, then fell till 3h 55m to
71.0 deg ; and afterwards rose till 4h 55m to
73-7 deg, the range being 3.4 deg, and at 4 inches
below the surface the t!mperature rose till 2h 50m
then fell till 4h 30m to 70.7 deg, '
a.nu
to 73 deg,
.2 deg, the range being
again rose till 6 p.m. to 73
2.5 deg.
The greatest cold on the ground occurred between