202 On Solar Radiation. 



the method of operation can be hardly made intelligible 

 except by shewing the experiment, and I merely allude to it, 

 because I have heard it asserted that these instruments are 

 useless when once injured. In my own attempts to repair 

 those in my possession I failed for many days ; but half an 

 hour was quite sufficient for the operation after the method 

 was discovered ; and what I have done, others can of course 

 hit upon, if they try to acquire manipulation from the above 

 brief hints. 



Thermometers with blackened balls seem to be best for 

 use in this country. Dr. Christie has described a very ex- 

 cellent form for one; but common thermometers may be 

 easily made use of. 



For this purpose, the best six inch scales should be se- 

 lected, and the hole round the ball should be enlarged to 

 at least one-fifth inch all round. The ball may then be 

 covered by applying any glutinous varnish and sticking 

 black wool or cotton to it, or, as I prefer, by painting the ball 

 with black sealing wax dissolved in spirits of wine. 



It is necessary that the black thermometer should be 

 covered with a close glass case to prevent the wind abstract- 

 ing heat from it. This is easiest done by enclosing the 

 whole thermometer in a thin glass bottle, or in a glass case. 



The case is made of common window glass, framed 

 together with narrow edgings of tin. It is ten inches long, 

 three and a half inches broad, and three inches high, with a 

 door at one end, and was made in the bazar, and cost a 

 rupee only. The thermometer is laid in it, supported on a 

 cube of wood about an inch each way, and the case is laid 

 on a teapoy resting on a book at each end, by which it is 

 kept free from contact with the wood. The time I prefer 

 for the observations is noon, when the sky is cloudless. 



The maximum effect of the sun's rays cannot be found 

 by leaving a thermometer exposed in the above case; for I 

 find that the air in it becomes gradually heated by the ra- 



