254 Mr. Colebrooke on the Climate of South Africa. 
Note on the preceding Essay, and on Meteorological Observations on the 
Atlantic, p. 115 and p. 241. 
Comparative trials made in the Laboratory of the Royal Institution, with 
Mr. Daniell’s hygrometer, and the hygrometer by evaporation, have satisfied 
me, that the stationary point of this last instrument is short of the dewing 
point. It becomes necessary, therefore, to correct the tabular diary (p. 134, 
et seq.), by substituting in the sixth column the head of ‘ Hyg. Stationary,’ 
in place of that of ** Deposition of Dew,” and to make the like change in 
the text (p. 117—119.) 
From observations with the hygrometer by evaporation (the dewing point 
being deducible from them), both the actual tension of vapour and quantity 
of moisture present may he computed. 
The following abridged table will serve to shew the correct point of dew, 
answering to observed depression of temperature by evaporation. The 
degrees of dryness in the seventh column of the tabular diary (p. 134, &c.), 
must be increased accordingly. 
TABLE, shewing the Dewing Point inferred from the Hygrometer, 
by Evaporation. 
Temp. | Hyg. 
Temp. . Dew. 
Atmos. | Evap. 
86 
