Page:Proceedings of the Royal Society of London Vol 60.djvu/147

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The density of aqueous vapour is a quantity having but a small diurnal variation, and it would appear, from a table published by General Sabine that the calculated mean potential for the day—taken as the mean of the calculated values for the 24 hours—would differ but little from that answering to only the two times, 10.30 and 4.30  Thus the calculated values in Table XXIII may be regarded as close approximations to the true calculated means for the seasons of the four observations. On the other hand, according to the table of diurnal variation of potential in the paper by Mr. Whipple already referred to, the true means obtained from observations at every hour of the day might be expected to be on an average some 10 per cent. higher than the observed values in Table XXIII. It ought, further, to be remembered that, as explained in § 4, the potential at station A must fall short of the true potential at a point 60 inches above the ground in the open, also the fraction of the existing potential picked up by the portable electrometer may be appreciably less than unity. Thus the fact that the calculated values in Table XXIII are so decidedly larger on the average than the observed is perhaps rather in favour of the formula than otherwise.

If we may judge, however, from the few data in the table, there seems some ground for the suspicion that the formula will prove to give too narrow a range.

Before concluding, I have much pleasure in acknowledging the ready and valuable help I have received from Mr. E. G. Constable, Senior Assistant at the Kew Observatory. Mr. Constable took all the electrical observations and the measurements of the meteorological curves, and gave me in addition much useful information derived from his long experience of the working of the electrograph and portable electrometer.