The Locomotive - Lighthouse Survival Blog

158 THE LOCOMOTIVE. [October,
is 122.29°. This shows that the quantity of heat required to raise the temperature of a
pound of water one degree is not the same at all points of the thermometer scale, but it
indicates, at the same time, that the total variation between the freezing point and the
boiling point is very slight. Similar experiments, made by mixing water at other tem-
peratures, give this same result; and hence we conclude that although a " heat unit " is
not the same at all temperatures, the variation in its value is so slight that for most pur-
poses it can be neglected. In calculating tables of the properties of steam and water,
allowance is made for the fact that the " specific heat " of water is not the same at all
temperatures; but in all the ordinary work of testing boilers and engines, and laying
out steam-plants and heating-systems, it is customary to consider the specific heat of
water to be constant; and the "heat unit " is defined simply as "the quantity of heat
that will raise the temperature of one pound of water one degree."
When we say that a pound of steam, in condensing, gives out 967 heat units, we
merely mean that it gives out an amount of heat that would be sufficient to raise the
temperature of 967 pounds of water 1°, or 96.7 pounds of water 10", or 9.67 pounds
100°, etc. Similarly, when we say that a pound of good coal gives out 14,000 heat
units when burned, we mean that each pound of the coal can heat 14,000 pounds of
water 1°, or 1,400 pounds 10°, or 140 pounds 100°, etc. And finally, when we say
that the "mechanical equivalent of heat" is 779 foot-pounds, we only mean if a weight
of 779 pounds should fall one foot, the work it would do would be just sufficient to
raise the temperature of a pound of water 1°; and, conversely, if we could utilize (by
means of a steam engine or otherwise) all the heat that a pound of water gives out
when it cools 1°, this heat would be just sufficient to raise a weight of 779 pounds
through, a height of one foot.
The Recent Eruption of Kilauea.
This great volcano has been active for several months past, the principal character-
istic being a remarkable rise and fall of melted lava within the crater. L. A. Thurston
gives the following among other, particulars in the Pacific Commercial Advertiser. In
March, 1894, the lava had risen almost to the top of the crater, the rise being 447 feet
in 19 months.
On the evening of July 6th a party of tourists found the lake in a state of moderate
activity, the surface of the lava being about twelve feet below the banks. On Satur-
day, the 7th, the surface of the lake raised so that the entire surface was visible from
the Volcano House. That night it overflowed into the main crater, and a blow hole
was thrown up some 200 yards outside and to the north of the lake, from which a flow
issued. There were two other hot cones in the immediate vicinity which were thrown
up about three weeks before. On Sunday, Monday, and Tuesday. July 8th. 9th, and
10th, the surface of the lake rose and fell several times, varying from full to the brim to
15 feet below the edge of the banks.
On the morning of the 11th the hill was found to have sunk down to the level of
the other banks, and frequent columns of rising dust indicated that the banks were
falling in. The lake had fallen some 50 feet, through the escape of the lava by some
subterranean passage, and the wall of the lake formed by the hill was falling in at fre-
quent intervals. The lava in the lake continued to fall steadily, at the rate of about 20
feet an hour from 10 o'clock in the morning until 8 in the evening. There was scarcely
a moment when the crash of the falling banks was not going on. As the level of the