TRANSACTIONS OF THE SECTIONS. : 29 
he obtains a series of measures differing from the true height—those corre- 
sponding to the hottest hours being in excess, and those appertaining to the 
coldest hours in defect of the true height. He then ascertains the amount of cor- 
rection which, being applied to the mean sum of the readings of the thermometer 
at each epoch in each of those months, would bring out the true height. In this 
manner he obtains a table, showing what he calls the normal correction for each 
of the nine epochs of the day during the four summer months. There is good 
reason to believe that, in reducing barometric observations which are to be com- 
pared with Geneva and the St. Bernard, the application of the normal correction 
ascertained in the manner above stated will in general give truer results than those 
where this is not applied; but as it is obvious that the conditions of temperature 
at the moment when a given observation is made are constantly yarying from the 
mean of the corresponding day and hour, it follows that a further supplemental 
correction should be made on this account. 
To apply this further correction is a matter of no slight difficulty. The method 
employed by M. Plantamour is as follows. He obtains from the observations at 
Geneva and the St. Bernard (by interpolation when necessary) the elements corre- 
sponding to the day and hour of the observation which is sought to be reduced, 
and from these he calculates the height of the St. Bernard. The height so obtained, 
when compared with the measure which is derived from the mean of the readings 
for the same day and hour, as shown in his Table of normal corrections, furnishes 
a criterion by which to judge of the conditions with respect to temperature of the 
moment when the observations to be reduced were made. M. Plantamour thinks it 
not difficult to infer from the observations themselves, and from the general state 
of the weather at the time, whether the moment was one of atmospheric equilibrium 
or the reverse. In the latter case the observation is treated as one of inferior utility, 
to which a lower value should be assigned in the final calculation. Supposing, 
on the contrary, the observations not to betray a disturbance of equilibrium between 
the two stations, the deviation of the height, as calculated for that particular mo- 
ment from the height derived from the corresponding means, is the measure of the 
amount and sign of the supplemental correction corresponding to the moment of 
observation. 
Without entering at present into sundry points of secondary importance, the 
writer believes that, while it is at present impossible to clear the mode of dealing 
with this correction of some arbitrary elements, it is easy to adopt a system less 
cumbrous and less inconvenient, and at least equally accurate with that proposed by 
M. Plantamour. He finds that many of the observations which appear to ML Plan- 
tamour to be clear of anomalies arising from the disturbance of atmospheric equi- 
librium, show unequivocal traces of such disturbance. These anomalies can be 
eliminated only by comparing the observations in hand with many ditierent stan- 
dard stations, such as Milan, Turin, &c.; but, in the absence of direct evidence, the 
introduction of an empirical correction in the manner proposed is likely to lead to 
error. 
The writer proposes to deal directly with the correction for temperature upon the 
best information that is available in regard to each of the stations where observa- 
tions are recorded. He considers that the deviation of the thermometer at the 
time of observation from its mean height at the corresponding day and hour, is a 
tolerably accurate measure of its greater or less deviation at that time from the true 
temperature of the air freed from surface-radiation, and may therefore be taken 
with its proper sign for the supplemental correction. 
It is important that the comparison between Geneva and St. Bernard, made by 
M. Plantamour, should be extended to other stations near the base of the Alps, 
and for this, as well as other reasons, it is highly desirable that the observations at 
Milan and Turin should be made at hours which correspond with the Swiss 
observations. 
On the Extent of the Earth’s Atmosphere. 
By the Rey. Professor Cuarus, M.A., F.RS., F.R.A.S, 
The object of this paper was to show that the earth’s atmosphere is of limited 
