358 METEOROLOGICAL INSTRUMENTS. Appendix A. 



corrections for capacity and capillarity should accompany every 

 instrument, and simple directions, &c, in cases of trifling derange- 

 ment, and alteration of neutral point. 



The observations for temperature were taken with every precaution 

 to avoid radiation, and the thermometers were constantly compared 

 with a standard, and the errors allowed for. The maximum ther- 

 mometer with a steel index, I found to be extremely liable to 

 derangement and very difficult to re-adjust. Negretti's maximum 

 thermometer was not known to me during my journey. The spirit 

 minimum thermometers again, are easily set to rights when out of 

 order, but in every one (of six or seven) which I took to India, 

 by several makers, the zero point receded, the error in some 

 increasing annually, even to — 6° in two years. This seems due to a 

 vaporisation of the spirit within the tube. I have seen a thermo- 

 meter of this description in India, of which the spirit seemed to 

 have retired wholly into the bulb, and which I was assured had 

 never been injured. In wet-bulb observations, distilled water or 

 rain, or snow water was used, but I never found the result to differ 

 from that obtained by any running fresh water, except such as was 

 polluted to the taste and eye. 



The hours of observation selected were at first sunrise, 9 a. m., 



3 p. m., sunset, and 9 p. m., according to the instructions issued to 

 the Antarctic expedition by the Royal Society. In Sikkim, however, 

 I generally adopted the hours appointed at the Surveyor General's 

 office, Calcutta; viz., sunrise, 9h. 50m. a.m., noon, 2h. 40m. p.m., 



4 p.m., and sunset, to which I added a 10 p.m. observation, besides 

 many at intermediate hours as often as possible. Of these the 9h. 

 50m. a.m. and 4 p.m. have been experimentally proved to be those of 

 the maximum and minimum of atmospheric pressure at the level of the 

 sea in India, and I did not find any great or marked deviation from 

 this at any height to which I attained, though at 15,000 or 16,000 

 feet the morning maximum may occur rather earlier. 



The observations for nocturnal (terrestrial) radiation were made 

 by freely suspending thermometers with naked bulbs, or by laying 

 them on white cotton, wool, or flannel ; also by means of a ther- 

 mometer placed in the focus of a silvered parabolic reflector. I 

 did not find that the reflector possessed any decided advantage over 



