4 Professor Gardner on the Influence of the Dew-point 



deflected plays across the arc and tends to assist the admea- 

 surement. 



The thermo-electric pair and galvanometer can be made 

 an extremely delicate differential thermometer ; and from ex- 

 periments already made by Drs. Forbes, Ritchie, Draper, &c., 

 we are justified in stating that the degree of variation of the 

 astatic needles is very uniform for equal increments of heat, 

 in cases where the total amount of variation is as limited as 

 in the following. 



In obtaining the numbers of the tables, or the measures of 

 temperature, the pointed extremity of the pair was thrust into 

 the parts of the plant specified, care being taken to avoid 

 contact by the fingers with either the plant or thermoscope ; 

 the numbers given are the mean of at least five measures made 

 by forming and breaking the electric circle. The same pair 

 and galvanometer were used throughout, and the value of a 

 degree of the index equals two elevenths of a degree of Fahr- 

 enheit, or 1° F. = 5°' 5 galvanometer. It is well to observe here 

 that the whole of the junction of the thermo-electric pair must 

 be introduced into the plant, otherwise the current of electri- 

 city does not circulate freely through the length of the wires, 

 but passes round from the warm to the cold parts of the junc- 

 tion, forming a circle that does not include the galvanometer, 

 and therefore producing no deflection of the needles. 



The dew-point marked in the tables was taken immediately 

 before and after each series of measures, and if any difference 

 existed, the mean adopted. 



The height of the thermometer is marked both at the time 

 of the deposit of dew upon the exterior of a glass of iced water 

 and its vanishing. The drying power, which is Dr. Dalton's 

 expression for the difference between the dew-point and at- 

 mospheric temperature, is also marked in the tables ; and it is 

 well to remark, that that great philosopher has ascertained 

 that the amount of evaporation is the same for all temperatures 

 if the drying power be the same. 



The experiments were performed in the shade, every dis- 

 turbing cause, as currents of air, motion, &c. being avoided. 

 The thermometer hanging at the side of the galvanometer, 

 and the dew-point, &c, were all estimated at the same spot. 



Arum Walteri (foliis sagittatis) was preferred for experi- 

 ment ; because it was in this genus Lamarck, Sennebier, &c. 

 noticed the striking variations of temperature recorded in the 

 commencement of this section ; it moreover flourished in my 

 immediate neighbourhood, and was of convenient size to esta- 

 blish a complete series of measures upon. The plants were 

 dug from the marsh in which they grew, with several pounds 



