1856.] Report on the Magnetic Survey, 561 



3. A thermometer exposed to the sun with white bulb. It had 

 no brass scale, and a very thin capillary tube for the mercury, sur- 

 rounded by a larger glass tube, the ordinary form of thermometers 

 on the Continent. The advantage of this is that the instrument 

 indicates nearly without error the temperature of the mercury in 

 the bulb unaffected by the disturbing influence of appendices. But 

 even in this shape, a thermometer offers no absolute measure for 

 insolation, the whiteness of the glass forming the bulb, its colour and 

 transparency modifying very appreciably the apparent action of the 

 sun on different instruments.* 



4. A thermometer of a similar construction, with blackened bulb. 



5. A Kew standard thermometer, with thick glass tube, the divi- 

 sions being in the glass stem. It had its bulb blackened, and also 

 half of the mantle of the cylinder behind the divisions. This, as 

 well as the following, was placed on a large surface of black wood, 

 which getting heated all round, very nearly as much as the thermo- 

 meter itself, prevents an irregular loss of heat towards objects of 

 different temperature in its vicinity ; the presence of grass or gravel, 

 for instance, would otherwise affect the reading of the thermome- 

 ter. It may be considered as a good proof of the comparability of 

 the thermometers in such an arrangement, that the Kew standard 

 and the following thermometer, No. 6, stood very nearly alike. 



6. A boiling thermometer, every degree divided into 50ths, the 

 mercury begins to reach the divided scale only at 78 degrees C, 

 being intercepted by a second enlargement of the tube. Bulb black- 

 ened. The mercury contained in the capillary tube being here only 

 a very small part of the mercury under the black stratum, and be- 

 ing protected besides by a second outer glass cylinder against loss 

 of heat, I found this instrument the best for these experiments as 

 long as the insolation was hot enough to raise the mercury to the 

 divided part. Besides, the y-J-o^ °f ^ ne degree being read with 

 perfect accuracy, it showed very rapidly even the minutest changes 

 in the atmospheric conditions. 



7. The surface of the ground. The thermometer was phiced in 

 the reddish sand forming the general deposit in the plains of Hin- 



* For relative determinations for distanee, and for the annual and daily varia- 

 tions, careful observations with any thermometer would be very useful. 



4 D 



