26 PHILOSOPHICAL SOCIETY OF WASHINGTON. 
graduation of degrees upon a thermometer scale. These were made 
with a common vernier of a mercurial barometer. A mark on the 
vernier was placed at each tenth (by estimation) of intervals of .05”, 
10”, .15”, .20”, .30”, and .40”, marked on the limb, and the vernier 
read for each estimation. Over 1300 readings were made, and these 
showed little difference in the splitting to tenths for the last 5 inter- 
vals above, but .05” seemed too small for accurate work. 
Results and methods of observing the Alluard form of Regnault’s 
condensing hygrometer were given. It was shown that if the ther- 
mometer immersed in the liquid is placed quite near the plate where 
dew is to appear, there is little or no danger of the air as it passess 
into the liquid harmfully affecting the thermometer. The com- 
plaint of some that the dew is deposited in the air, having a tem- 
perature frequently 40 or 50 degrees above the liquid, and hence 
that the thermometer can hardly give a correct dew-point tempera- 
ture was shown to have little weight, since the results, with a slight 
difference between the air and dew-point, at which time the effect 
would be small, are nearly identical with those where the difference 
is large, and the effect would be large. It was shown that the great 
difficulty in nearly all psychrometric work up to the present time 
has been the disregard of a sufficient ventilation of the wet-bulb 
thermometer. The sling psychrometer, with a few precautions in 
its use, furnishes results entirely satisfactory. 
The comparisons so far made between the two instruments have 
shown a remarkable uniformity under all conditions of moisture and 
temperature, and have left little to be determined in order to make 
either apparatus one of precision. A probable effect of compression 
of ice on the wet bulb at temperatures of 0° F., and below, was 
shown to exist, though this may be due to the lack of conduction 
on the part of the ice for the residuum of heat in the bulb. The 
question of the effect of height above sea on the indications of the 
above instruments was touched upon, and it was shown that the 
effect was small and only to be detected by the most refined obser- 
vations. Also, that until we have some law for reducing humidity 
results to sea level, the propriety of introducing such effect into tables 
is questionable. 
Mr. T. C. MenpENHALL exhibited a new volt-meter devised by 
Sir William Thompson. The principal difficulty entountered by 
earlier instruments of this class has arisen from the inconstancy of 
the magnetic force of the terrestrial field. By this instrument ter- 
