350 
LIEUT.-COLONEL SYKES’S DISCUSSION OF METEOROLOGICAL 
bulb and of the dew-point, have been observed even in our damp climate at the 
Royal Observatory at Greenwich. For instance, in the records on the 6th of April 
1845, at 6 P.M., the dry bulb was 52°-6, wet bulb 39°'2, and dew-point by Daniell’s 
hygrometer 22°. The depression of the wet bulb being 13°'4, the dew-point by 
Apjohn’s formula would be 14°, by Glaisher’s factors 25°-8, and by Daniell’s hygro- 
meter it is found to be 22°. Which of these dew-points is to be taken to give the 
real numerical value of the tension of vapour ? At Greenwich, on the 3rd of June 1846, 
at 4 P.M., the dry bulb was 79°' 1, and the dew-point by Daniell’s hygrometer 44°, the 
depression therefore 35°' 1. But I have recorded in the Meteorology of the Deccan, a 
depression of 61° of Daniell’s hygrometer taking place before the deposition of 
dew ; the temperature of the air being 90°, the dew-point 29°, at 4 p.m., on the i6th 
of February 1828, at Downd, near Pairgaon, on the Beema River*. Mr. Glaisher, 
of the Royal Observatory, a most persevering and able observer, finding from expe- 
rience that the formulae in use with a constant coefficient did not give satisfactorily 
the dew-points at varying temperatures and varying depressions of the wet bulb, 
adopted a series of factors quite independent of theory, from the results of very ex- 
tensive comparisons of simultaneous observations of the wet bulb with Daniell’s 
hygrometer. These comparisons extended over several years, and through several 
thousand observations. He found that the dew-point temperature was so related 
to the temperatures of air and evaporation, that at the same temperature of the air, 
the difference of temperatures of air and of the dew-point, divided by the difference 
of the temperature of the air and evaporation, was constant, but that it was different 
at every different temperature.” Arranging several thousand observations made 
during five years at Greenwich, and during three years at Toronto, and taking the 
mean value at every degree of temperature, he obtained a series of factors ranging 
from 8 5 at temperatures below 24°Fahr., up to 1'5 above 70° Fahr. ; these were 
published in the volume of the Greenwich Magnetical and Meteorological Observa- 
tions for 1842, 1843 and 1844. In 1847 Mr. Glaisher published hygrometrical 
tables in which some slight alterations of the original factors were made, and he has 
since made some further slight modifications. For the limited range in temperature, 
depression of the wet bulb and pressure, in w'hich the comparisons were made and 
the factors deduced, they are probably the very best agents (at least above the 
freezing-point) for giving a close approximation to the true value of the readings of 
the wet bulb ; supposing always the wet bulb not to be subjected to the anomalous 
indications noticed by Professor Orlebar and M. Regnault. For considerable dif- 
ferences of barometrical pressure, and very considerable depressions of the wet bulb, 
the factors would require further correction. When the dew-point was obtained 
with difficulty by Daniell’s hygrometer, or in other words, when the depression 
of the temperature of evaporation was great, Mr. Glaisher found that his method of 
determining his factors would not hold good, which he attributed to certain objec- 
* Philosophical Transactions, Part I. for 1835, p. 184. 
