518 
To sum up, the genus Hypolimnas is dis- 
tinguished among Nymphaline genera for 
the extent to which its numerous and wide- 
spread species resemble the local distasteful 
forms of Hupleeince or Danaine. 
Upon the older theory of Bates this 
would be explained by supposing that the 
genus is very hard-pressed in the struggle, 
and has thus been driven to mimicry 
almost everywhere. Upon the newer Mul- 
lerian theory it is supposed that the genus 
is distinguished among Nymphaline genera 
by some special defense, probably in the 
way of taste or smell or indigestibility, and 
that it has been to its advantage to adopt 
the advertisement of still better known and 
probably still more distasteful forms in its 
locality. 
The abundance of the various species, the 
conspicuous nerina form of female, and the 
resemblance of a rare Danaid to it, the 
recent spread of H. misippus beyond the 
limits of its model, all support this latter 
interpretation. 
NOTES ON ENGINEERING. 
Tuer cost of power in New England cot- 
ton mills has been, of late, the subject of 
some discussion in technical and lay jour- 
nals. The lowest cost yet reported, with 
one exception, is that given by Mr. Sheldon 
for the case of a mill which, paying $1.76 
per ton for coal, obtained the horse-power 
for a total cost per annum, including all 
items on the treasurer’s books, interest, 
depreciation, taxes, etc., of $11.64. 
This figure was challenged and compared 
with the items generally given for other 
classes of engine which are usually two or 
three times as great and often much more. 
But the latest report comes from the War- 
ren Steam Cotton Mill, where an engine of 
1,950 horse-power, a cross-compound con- 
densing machine, with cylinders 32 and 68 
inches diameter and of five-feet stroke of 
piston, making 74 revolutions per minute, 
SCIENCE. 
[N. S. Von. VI. No. 144. 
steam at 155 pounds at the boiler, supplies 
power at the cost of 1.35 pounds of coal 
per horse-power hour. The engine was 
designed by Edwin Reynolds, the boilers 
built by the Heine Company. The follow- 
ing are the figures certified to Dr. Thurs- 
ton by the treasurer of the mill. The en- 
gine replaces a quadruple-expansion engine, 
destroyed by fire, after seven years of ex- 
cellent service. The change illustrates 
the fact that the cost of the higher grade of 
machine may more than compensate its ,ex- 
ceptional economy; a fact which has only 
in late years come to be recognized. 
In the following table of the costs of the 
new engine the figures come from the treas- 
urer’s books. Coal costs $2.26 per ton, and 
in the account includes all costs of all steam 
used for all purposes, including banked 
fires, nights and Sundays, and that sup- 
. plied the mill. 
The following is a tabulated statement 
of the cost of power : 
Fuel per horse-power per year of 3,070 hours...$ 4 70 
TEAR rocco sacnodacboobcoscdbanNbooEHHOORDAEboRSCAOADSOAOCO 1 88 
Supplies and repairs...........cseeeeeseesseeseereeenes 42 
Total operating eXPeMSes ...........esseseereeeeees $ 7 00 
Interest at/5 per Cent.............00scceeseecossereoees $ 2 05 
Depreciation, at 5 per Cent............sececeeeeeeeees 2 05 
AINE BRE Sb acpnonposnnonasogancbotooansoquanacnSHnoonoociabAoCS bon 41 
ISTHE AVEC agonsoococenpabansnoco ono Sab SononSSnaNoeooEDOSI08 04 
LTS GBR Soc sqnsaobono sono sndoNoASvENEanaGEGAsHGI $ 4 55 
Totals cost of power per yeal............eeeeeeeee $11 55 
According to the Providence (R. I.) Jour- 
nal: ‘‘ This is lower than anything yet found. 
It is due to the large size of plant, which 
reduces the labor and supply account per 
horse-power, and to low cost of fuel and in- 
surance and low cost of plant, on account of 
its size. The cost of plant includes a Green 
economizer, chimney, boiler-house, engine- 
house and foundations—all first class— 
and water-tube boilers, whose depreciation 
ought not to be over 24 per cent. If steam 
used for other purposes than power were 
deducted, it would reduce the fuel 10 per 
