430 JOURNAL OF THE PLYMOUTH INSTITUTION. 
example 1869, when it was 72° F. The months of greatest range 
are May and June; the months of least range are December and 
January. The greatest daily range of temperature I have ever 
recorded was 40° F., on 27th April, 1865—the maximum, 86°, and 
minimum, 42°F, As a contrast to this, on December 22nd, 1870, I 
found the daily range only 2°—the maximum, 28°, and minimum, 
26°—-and on several occasions I have found but 3° range, the last 
being 14th November, 1880. Great daily ranges are indications 
of dry weather, because the screen of vapour which I spoke of just 
now does not exist, or, in other words, there is but little moisture 
in the atmosphere; but small daily ranges show an atmosphere 
saturated with vapour which a small decrease in temperature will 
precipitate in the form of rain. The connection between great 
daily ranges of temperature and health is an interesting problem 
for some medical man with time on his hands to solve. Another 
problem is, Do extreme temperatures, even if they last for some 
days, affect health as much as great daily ranges of temperature do? 
Tf so, in what way? The average temperature at 8 a.m. is 50°82° F., 
which is within ‘88° F. of the mean for the year. During only the 
months of May, June, and July is the 8 a.m. higher than the 
average ; hence the mean temperature for the day must be after 
8 a.m., and probably between 9 and 10am. ‘The gradual rise and 
fall of the temperature from month to month is shown by the 
curve on the diagram, where it is seen that the rise during the first 
months is much more gradual than the fall during the latter months 
of the year. Our mean maximum is 58°18° F. ; our mean minimum, 
45-05° F. ; and our mean temperature, as I stated before, 51°62° F. 
Hy@romMetry AND RainratL.—LBy hygrometry is meant the 
estimation of the amount of aqueous vapour in a given volume 
of the atmosphere. There are several ways of performing this 
estimation, and hygrometers have been invented by Daniel, 
Regnault, and others, for that purpose ; but the method generally 
adopted by meteorologists is that suggested by Leslie, and the 
form in general use is due to Mason—hence is generally called a 
psychometer, or Mason’s hygrometer. It consists of wet and dry 
bulb thermometers, from the readings of which, and from em- 
pirical tables constructed by Glaisher, can be obtained the dew 
point, the saturation of the atmosphere, the quantity of vapour in 
a cubic foot, and other data. Evaporation produces cold, because 
