316 Daniell on a new Hygrometer. 



arises the probability of falling weather : aud, according as it 

 happens, by the fall of the temperature of the air, or by the rise 

 of the constituent temperature of the vapour, so is the probabi- 

 lity of its less and greater quantity, its shorter or longer conti- 

 nuance. The thirteenth and fourteenth columns contain the 

 maximum and minimum of temperature in the course of the 

 twenty-four hours, as marked by a register thermometer. The 

 fifteenth column shews the lowest temperatui'e of a thermometer 

 laid upon the ground during the night, with its bulb covered 

 with dark wool. The late Dr. Wells, in his admirable Essay 

 upon Dew, has shewn that the quantity of that nocturnal preci- 

 pitation of moisture upon different bodies is ccBteris paribus 

 proportionate to the quantity of heat which they radiate to the 

 sky. My object, in this division of my register, is to obtain the 

 lowest temperature of a good radiator, and of one that approaches 

 nearly to the nature of grass and other vegetables in that parti- 

 cular; as the knowledge of this point; connected with that of the 

 actual quantity of vapour in the air, may furnish data upon 

 which to form an estimate of -the actual amount of the aqueous 

 deposition. As connected with this subject, I may here remark, 

 that the average temperature of the morning vapour exceeds 

 that of the night by about 1°, making a difference in quantity of 

 about 0.138 grain in the cubic foot less in the night than in the 

 morning. The sixteenth column registers the quantity of rain 

 at different periods. 



The seventeenth contains an account of the variations of a 

 De Luc's hygrometer. This I have extracted from the London 

 Medical Repository. It is furnished by Messrs. Harris and 

 Co., of Holborn ; and, on account of the vicinity of the places of 

 observation, will very well answer my purpose. It is perhaps, 

 upon the whole, the best instrument of the kind that was ever 

 invented, and it will be seen how vague and indecisive its indi- 

 cations are. 



The eighteenth column shews the force of evaporation in the 

 number of grains which would rise in one minute from a vessel 

 of six inches diameter. 



The direction of the wind is exhibited by the nineteenth 



