102 Dauiell on Meteorological Observations. 



mating the force of evaporation from the results of the hygro- 

 meter appears to me to be fully as correct as that in common 

 use, and, if any method could be devised of measuring accurately 

 the velocity'of the wind, would be infinitely preferable. The ca- 

 pacity of the atmosphere for moisture at any given time is mea- 

 sured ; the rapidity with which that capacity is saturated is 

 dependant upon the temperature of the body which yields the 

 moisture. These conditions vary almost ad iiifinitum. They 

 vary on the land and on the water, they vary in sunshine and in 

 the shade, they vary as land is more or less clothed with 

 vegetation, or as water is more or less deep. The hygrometer 

 is capable of following and appreciating all these changes, but 

 the common gauge can only give the amount of evaporation 

 from the shallow body of water in the place where it has been 

 fixed. After all, the estimation of the mean evaporation from 

 the surface of the earth is the most difiicult problem of metero- 

 logy. From the sea, and other deep bodies of water, whose 

 temperature is tolerably equable, the process is pretty regular ; 

 but one part of a field exposed to the sun is often yielding 

 moisture at the rate of eight or nine grains per minute from a 

 circular surface of six inches diameter, while another part is 

 only yielding one grain and a half. 



The eighth and ninth columns contain the quantity of rain, 

 and the means of temperature, measured and calculated in the 

 usual way. In Table 2, are inserted the mean results of both 

 years in small figures, and the general mean in large. My 

 great object has been to study facility of comparison, and I 

 purpose, if I should continue my task, to collate in such a 

 table the results of the current year, with the general average of 

 all. This average will increase in accuracy as we proceed, and 

 will, in a course of years, furnish a perfect standard by which to 

 judge of the progress of the seasons. I am happy to find that the 

 results which I have obtained witli the barometer and ther- 

 mometer correspond very closely with those deduced by Mr. 

 Howard from a long series of years. 



Table 3 is a general table of temperature, in which I have 

 collected some particulars which were never before attended 



