176 
POPULAE SCIENCE I^EWS. 
[Decembkr, 1891. 
sun (AlpJia Aurigse, for instance, ) H and K are 
about alike, and in some cases K is even the 
strongei- of the two. Tlie matter is now clear ; in 
the spectrum of Sirius and its congeners the band 
at H is due to hj^drogen, while in the spectrum of 
Capella it is mainly due, not to hydrogen, but to 
the same substance that produces the H and K of 
the solar spectrum. 
These bands, as has been said, are generally at- 
tributed to calcium, and this view may be correct ; 
but a number of facts make one hesitate at accept- 
ing the identification as final. It is true that the 
spectrum of the metal in the electric arc presents 
two lines that coincide perfectly witli the center 
of the two bands; but the other lines of calcium, 
which in our laboratory experiments are more 
conspicuous than these, seldom present them- 
selves in the spectrum of the prominences, and 
■when they do, only faintly, while 11 and K incom- 
parably outshine the lines of hydrogen itself, and 
attain higher elevations. Moreover, in the photo- 
graphs of the spectrum of the corona obtained 
during the eclipse of 1882, these lines dominate 
everything. The facts seem to indicate a gas 
lighter than hydrogen itself, and almost compel 
the belief that if they are really due to calcium- 
vapor it must be calcium in a very different condi- 
tion from any with 
which we are acquain- F' "" "^ 
ted in our laborato- 
ries. But it is perhaps 
quite conceivable that 
under solar conditions 
the metal may assumt; 
an allotropic state in 
which its vapor-den- 
sity becomes very low. 
Mr. Hale has taken 
a further step in util- 
izing these two lines 
to enable him to pho- 
tograph the promi- 
nences themselves, 
and with very fair suc- 
cess. The idea is not 
new; some twenty 
years ago the w liter 
succeeded in getting a 
photographic impres- 
sion of a prominence 
by using a spectro- 
scope with a widely 
opened slit, and working through the hydro- 
gen line near G in the blue part of the spec- 
trum. The trial made it obvious, however, 
that no practical result could be reached by 
the photographic processes then in vogue. But 
these bright H and K lines of the chromosphere 
spectrum, falling as they do, each in the center of 
a wide, dark space in the solar spectrum, are pho- 
tographed with the greatest ease ; and by opening 
the slit of the spectroscope it is possible, if the 
clock-work of the telescope runs accurately, to se- 
cure a really good picture of the form and struct- 
ure of quite a large prominence. By fitting the 
instrument with a mechanism devised by Mr. 
Hale, such that the slit can be carried across the 
prominence while the sensitive plate is protected 
by a moving diaphragm, except where the image 
of the slit falls at the moment, it is possible to 
cover a much wider area with the picture. In 
fact, by an arrangement devised by Deslandres, 
one could photograph in a single operation the 
chromosphere and prominences around the whole 
circumference of the sun. 
College of New Jersey, PRiNCETO>f, Nov. 16, 
1891. 
THE ASTRONOMICAL SIGNIFICANCE OF 
STONEHENGE. 
There is no more interesting or more mysteri- 
ous pre-historic monument in Great Bi'itain than 
the wonderful group of " standing stones" which 
form so conspicuous an object upon Salisbury 
Plain. Volumes have been written as to the age 
and purpose of this monument, but absolutely 
nothing is certainly known. It was erected 
in an age and by peoples that have passed en- 
tirely out of human memory. The popular tradi- 
tion considers Stonehenge to be a temple of the 
ancient Druids, but there is no historical basis for 
the belief; and, on the other hand, there is noth- 
ing to show that it might not have been such a 
^ temple. 
Only a portion of the stones are standing in 
their original position at the present time, but 
enough are left to give us a good idea of the origi- 
nal plan of the edifice. It was composed of about 
130 stones, and the general plan was that of a 
double circle of stones inclosing two ovals, which 
formed, perhaps, the sanctuary, or holy place, of 
the temple. The great circle was composed of 
enormous perpendicular stones, about twenty feet 
high, supporting a row of horizontal ones, as 
shown in the engraving, which are secured in 
their place by well-formed mortises and tenons. 
All the stones have been quarried and shaped by 
means of tools. Those of the largest circle are 
composed of sandstone occurring in the vicinity, 
while the stones of tlie interior circle are of a dif- 
ferent nature, and must have been brought from a 
considerable distance. 
In the center of the edifice is a block of stone 
sunk low in the ground, which is popularly — and 
probably correctly — called the altar stone. At 
some distance to the northeast, and standing out- 
side of the circle, is a sep.arate monolith known as 
the "Friar's Ileol." It li.is always been stated, 
as showing the asti-onomical knowledge of the 
builders of Stonehenge, that on the 21st of June, 
or the summer solstice, the rays of the rising sun 
shine directly over the Friar's Heel and fall upon 
the altar stone in the center. If this were true it 
would show that Stonehenge was built by a peo- 
ple well advanced in civilization, and add weight 
to the theory that it was a temple connected with 
the ancient sun worship. 
To prove the truth of this belief two English 
gentlemen, Messrs. Bacon and Howe, made a pil- 
grimage to Stonehenge on the night preceding the 
21st of last June, and, placing a camera on the 
altar stone facing the Friar's Heel, patiently 
waited for dawu. As the sun appeared above the 
horizon the exposure was made, and the old tradi- 
tion verified. .\s shown in the engraving, — which 
is a copy of the photograph, and reproduced from 
La Nature, — the sun, as seen from the central 
altar, appears to rise directly over the exterior 
monolith. Evidently the ancient builders of this 
remarkable edifice' must have had considerable 
astronomical knowledge to determine the time of 
the summer solstice so accurately. 
Whatever may have been the history of Stone- 
henge, it remains one of the most interesting 
monuments of the Old World. It is a sight which 
no traveller in England can afford to pass by. 
Whatever its original design, whether for a sepul- 
chral monument, temple of worship, or astro- 
nomical observatory, it indicates the expenditure 
of an immense amount of time and labor on the 
part of its pre-historic builders ; and to the writer, 
who visited it some years ago, the ruins of this 
rude but massive structure, standing on the deso- 
late plain, and surrounded by innumerable burial 
mounds and pre-historic tumuli, were far more 
impressive than the cathedrals and castles of 
more modern times, which are so .abundant in 
Great Britain, and 
were a striking and 
visible illustration of 
the impenetrable mys- 
tery which surrounils 
the origin and early 
liistory of the human 
race. 
o 
IIORSE-POWEK.— To 
an inquiry how to 
compute the horse- 
power of an engine, 
the Scientific Ameri- 
can replies : Multiply 
the square of the di- 
ameter of the cylinder 
l)y 0.7854, and this 
pioduct by the mean 
pressure in the cylin- 
der. The mean pres- 
sure, assuming the 
usual practice in 
small engines at five- 
eighths cut-ofl', will 
be 0.92 of the boiler pressure. Multiply the last 
product by the speed of the piston in feet per 
minute and divide by 33,000 for the horse-power. 
[Original lu Popular Science News.] 
LOCUSTS. 
BY S. L. CLAYES. 
All our lives we have heard of the likeness 
which the locust hordes bear to au immense army, 
advancing with a force so overwhelming as to 
defy resistance. We have been familiar with the 
dread but sublime description of their coming 
given by the prophet Joel. We have even had 
some experience — though happily slight — of their 
ravages in our own country. But however vivid 
may have been the descriptions we have read or 
the tales to which we have listened, it is only l)y 
the added aid of a powerful imagination that we 
of the Western world are enabled to form any 
just idea of what a swarm of locusts really means 
to the nations of the East. 
Dr. William Thomson, in his .account of one of 
their incursions into Syria, tells that early in the | 
season a detachment of winged locusts passed ; 
