1821.1 Mr. BlackwaU's Thermometrical Observations. 296 



Half-past eight and half-past ten, a. m. and half-past ten, p.m. ; 



mean 56-40°. 



Half-past eight and eleven, a. m. and half-past eleven, or 

 twelve, p. m. ; mean 56-40°. 



Half-past seven, a. m. and half-past six or seven, and eleven, 

 p.m.; mean 56-40°. 



Half-past seven, a. m. and half-past seven and half-past nine, 

 p. m. ; mean 56-40°. 



Eight, a. m. and half-past seven and ten, p. m. ; mean 56-40°. 



Half-past eight, a. m. and half-past six or seven, and half-past 

 eleven or twelve, p. m. ; mean 56-40°. 



Half-past eight, a.m. and half-past seven and half-past ten, 

 p. m. ; mean 56-40°. 



Half-past eight, a. m. and eight and ten, p. m.; mean 56-40°. 



Ten, a. m. and half-past nine and half-past ten, p. m. ; mean 

 56-40°. 



The extremes by Six's thermometer were 46-3° and 65°, the 

 mean of which is 55-65° ; and the extremes by Rutherford's 

 thermometers were 47° and 66-5°, the mean of which is 56-75°. 



It is very probable that the mean temperature of the natural 

 day is generally taken too high in summer ; especially when 

 found from three observations, one of which is made at one 

 o'clock, p.m. The most convenient method of taking it, and, 

 perhaps, as accurate a one as can be adopted, having the addi- 

 tional advantage also of giving the extremes of temperature for 

 the period, which are not obtained by the usual modes of observ- 

 ing, is to find the mean of the daily extremes registered by a 

 good Six's thermometer, or by a pair of Rutherford's horizontal 

 thermometers. This result will not be far from the truth. A 

 preference, however, will generally be given to Rutherford's 

 thermometers, as they are rectified by simply raising one extre- 

 mity of the frame to which they are attached, and letting the 

 indexes slide, one to the end of the spirit, and the other to the 

 end of the mercury ; whereas the application of a magnet is 

 necessary to bring the indexes of a Six's thermometer into their 

 proper situations. 



For the more perfect attainment of the object under consider- 

 ation, another series of observations similar to the foregoing 

 should be made in the month of December. 



