The National tribune. [volume] (Washington, D.C.) 1877-1917, 1890-10-23
CARINATED SCALES
THE WEATHER.
Dynamic Heating and Cooling Of the Air.


Readers OF Tyndall's interesting lectures on
Heat a Mode of Notion" will recall two
striking and paTadoxical experiments, in which
by projecting u current OF air on the face OF
thermople a heating effect was manifested in
one case and cooling effect in the other. In
the first, air was forced from a bellows; in
the second, air which had been condensed in
strong metallic vessel, and in its compressed
state had acquired tho temperature of the sur.
rounding air, was allowed TO escape and produce
blast on the face of the pile. Tho solution
OF the paradox lay in the fact that air from
the bellows only differed from that around by
being in rapid motion, aud this motion, when
arrested by the thermopile, was converted into
heat. In the case of the compressed air, tho
escaping air, before reaching the theTmopie
had done work in the act Of expanding by
overcoming the pressure of the atmosphere
and thrustiDg IL aside and the heat expended
in performing this work. aud therefore lost
was greater than that reproduced from tho ar
rested movement. These two experiments are
instances of dynamic heating aud dynamic
cooling
Whenever air or, indeed, any other sub
stance, in expanding has 10 overcome resist
ance the work done IL represented by loss of
heat, not necessarily by fall of temperature,
because IL the expansion takes place In conse-
quence Of heating undera constant pressure the
temperature must be rising at the same time.
Bat the quantity OF heat which IL absorbs nn.
der these circumstances is greater than would
be necessary 10 raise IL 10 the Same tcmpera.
ture were 10 not expanding against resistance
This difference Is that of the specific heat of
air at constant pressure, viz: when expanding
and at constant volume. The specific heat of
air Is 02375. or the quantity OF heat required
10 raise a certain weight Of air ono degree of
temperature as compared with the standard Of
water whose specific heat Is !. The specific
heat Of water and other substances varies to a
small extent at different temperatures, but for
gases the change is very small. and for such
ranges as We have to deal with in metoorology,
We may assume the specific heat as constant for
tue same substance ia the same physical state.
When the air, while being heated, is con-
tanned in q close vessel no work Is done, and the
| heat absorbed goes simply TO raise It's tempera-
! ture But If IL expands and remains OF the
| same elastic pressure, portion Of the heat alb
! sorbed Is used up in making room for tho ex-
/ pansi0n by thrusting aside or lifting the mass
/ that presses upon IL. The ratio Of this excess
| TO the whole quantity of heat absorbed is con-
/ slant, aud therefore tho specific heat at con,
slant pressure bears a constant ratio TO that M
constant volume, viz: 1.4l to !. On the other
hand, air which is cooling, and therefore losing
| elastic force under constant external press
ure, has to get rid not only Of It's actual heat
but OF that which is developed In 10 by til,
compression it uudergoes. That air which i,
condensed by any increase Of external pressure
becomes heated Is a fact familiar 10 any one
who has ever compressed air by means OF Il con
densing syringe, as for instance in charging
| the reservoir Of an air-gun.
The great discovery of Joule that the fal
OF mass Of matter weighing TIP pound
through hight Of one foot will develop, whci
It's motion Ia arrested, sufficient heat TO rails
one pound Of water nedegrce Fahrenheit, viz
one unit Of heat, affords the means of comput
ing the exact quantity of heat that Is devel
oped by the'c0mpression disappears in Tia
expansion Of air under given pressure. Tn
mechanical equivalent Of one unit of heat bein
772 fo0t-ponnds. If we multiply tho pTessur
ESTIMATED in pounds on the square FOOT b,
the hight through which it falls In the on
case or is lifted in the other correcting the re
suit for any difference in gravity and divid
this product by TIE, we obtain the number q
heat units that are produced or disappear
Dividing this by the specific heat OF air at con
slant volume, aud also by the weight in pound
Of the air compressed or expanded, we obtai
tue number Of degrees through which it wi
be heated in the one case and cooled in til
| other.