On Solar Radiation. 201 



observations made by Mr. Dalmahoy at Moulmein, (Madras 

 Journal, vol. iv. page 55,) are of doubtful nature. 



In Reid's Chemistry also, (third edition, page 678,) it is 

 erroneously stated, that the "actual influence of the sun's 

 rays is measured for one minute by subtracting the mean 

 of two indications in the shade from the amount induced 

 on direct exposure to the sun ;" an error probably caused 

 by persons not accustomed to the use of algebraic symbols 

 mistaking the application of the signs of the quantities. 



Dr. Reid also is in error in saying, that the instrument 

 " measures the action of the sun's rays apart from the cool- 

 ing influence of surrounding objects;" for it obviously only 

 measures the difference between the heating effect of the 

 rays, and the cooling effect of radiation from its surface, and 

 of the contact of the atmosphere. 



Leslie's Photometer is another instrument for this pur- 

 pose ; but unfortunately, it is so liable to get out of order, as 

 to be almost useless in this country; because, if inverted 

 during carriage, the thread of the coloured liquor becomes 

 broken. The instrument may, however, be again put in order 

 by holding a piece of hot charcoal near one of the balls, 

 which will drive all the fluid into the other, and thus re- 

 unite it. 



To adjust the zero point to the scale, the charcoal is then 

 to be applied to the ball containing the fluid, by which it 

 will be driven over into the other ball, and then the bubbles 

 of air may be passed on through it one by one, until, when 

 the instrument is allowed to cool, the top of the thread of 

 fluid will remain near the zero point. During the operation 

 bubbles are apt to form, which cause great trouble; but by 

 getting them into the top of the ball, they can be broken by 

 suddenly bringing the hot piece of charcoal nearly into con- 

 tact with the glass, by which a sudden expansion of the 

 bubble is produced, which will cause it to burst, while the 



thinness of the glass will prevent its being injured. I fear 



2 D 



