301 



computed by my assistant, from your last formula, to which 

 I have added a vapour table, computed by Biot's formula, 

 from Dalton's experiments, which is the same that Pouillet 

 has used in the last edition of his Traite de Physique, that 

 I have seen. Every one of the numerical values in this 

 table (No. 3) has been computed directly from the formula 

 to seven places of decimals, five of which only have been 

 retained. In like manner, in part 1, each of the numerical 

 values for depressions to one-tenth of a degree Fahrenheit, 

 has been directly computed for twenty and for thirty inches of 

 pressure, and the intermediate values obtained by addition. 

 All the work has been checked by diflferences, and examined 

 by three separate computors. 



" Accompanying, are also sent the separate observations 

 of the wet and dry bulb thermometers, of Daniell's hygro- 

 meter, and of the standard barometer, with notice of the 

 weather for the twelve term days of 1841, which have been 

 taken specially for your own use, and which you will, I think, 

 find to confirm the views you had already taken upon the 

 subject. There is one point which strikes at first sight, viz., 

 that with very few exceptions, the dew-point, by the hygro- 

 metei-, is too high. It is a difficult instrument to use. It 

 requires the observer to approach near to it ; and, even with 

 the utmost care, it is difficult to prevent an effect sensible at 

 times to a prejudicial extent, upon the hygrometric condition 

 of the surrounding air. None of these difficulties occur with 

 the wet and dry thermometers, which have one still greater 

 advantage, viz., the ease with which the wet ball can, under 

 almost any circumstances, be moistened. 



" During several months, we observed the hygrometer 

 hourly on term days, but finding that this gave much trouble, 

 and was likely to pi'ejudice the readings of the magnetome- 

 ter, I discontinued the practice, and had since but one ob- 

 servation every two hours. 



During this year, I regret to say, that a failure in ether 



