IN METEOROLOGICAL REGISTRATION. 89 
and rises in the afternoon. To contribute to the attain- 
ment of this object I have shown in the table the 
difference between the temperatures of the wet and of the 
dry bulb thermometers during the day, which difference 
expresses the energy of evaporation. Jn all meteorological 
observatories this should be attempted. The extent to which 
the wet bulb instrument is kept below the dry one shows 
the proportional amount of vapour sent into the air. And 
the hourly heights of each instrument may, during the 
day, be easily worked on paper ruled to a scale of degrees 
of temperature. By drawing a straight base line on such 
paper to represent the wet bulb thermometer, then dotting 
the height of the dry above the wet one at each hour, and 
connecting the dots by lines, there would be presented 
to the eye a curve, showing the relative quantities of 
vapour that ascend into the atmosphere during each hour. 
It would be easy to compare these curves of evaporation 
with others, representing the mid-day falls of the baro- 
meter, with a view to tracing their connection. If the 
same kind of registration were to be made when storms 
occur in any part of the world, it would almost certainly 
throw new light on the cause of storms. 
The movements of the barometer, as an indicator of 
atmospheric pressure, are well known, but the influence of 
vapour, while undergoing condensation into water, on that 
pressure, is not known, and therefore to this point atten- 
tion should be strongly directed. As long as the fresh 
vapour of the morning shows itself only in an increase of 
tension it may be considered to produce no disturbance of 
the gaseous atmosphere, though it may raise the barometer. 
It will, therefore, then be sufficient to register the wet and 
dry bulb thermometers, the tension of vapour, and the 
barometer, as these will exhibit what is taking place during 
the time. But when the barometer begins to fall at nine 
or ten o’clock the difference between the two thermometers 
VOL, XV. N 
