1875.] Suggestions for the Yunan Expedition. 5 



5. "Wind and cloud observations have already been referred to. Any 

 opportunity which may occur of observing the local variations of the moun- 

 tain and valley winds should be taken advantage of. The diurnal changes 

 of the wind in the valleys and passes are of special interest. Any observa- 

 tions of the night winds which blow down the valleys, the time they set 

 in, their duration, force, and temperature will be important. Also their 

 upper and lower limits. 



It might also be worth while, if possible, to make local enquiries regard- 

 ing the seasons, the prevailing winds, and the rainfall. 



Barometric readings will probably be taken on the march for the de- 

 termination of heights. As connected with the subject of meteorology it 

 may not be out of place to remark here that whether atmospheric pressures 

 be inferred directly, for this purpose, from the readings of a barometer, or 

 indirectly from the boiling temperature of water, the observations should be 

 made with a view to determine relative heights, or differences of heights, 

 and not absolute altitudes above sea level. The absolute height of a given 

 place above sea-level can only be determined from means of barometric 

 observations, with any approach to accuracy, when the mean pressure of 

 the air in the particular latitude, and for the particular time of year at sea- 

 level, is known and also the direction and magnitude of the atmospheric 

 gradient. But differences of heights may be calculated from clay to day, in 

 serene weather, by comparing obervations of the pressure of the atmo- 

 sphere (whether determined by barometer, or boiling point, the former of 

 course being the more accurate) made at the same hours on successive days. 

 It is obvious that only observations made at the same hours can be com- 

 pared, as the difference of pressure between the 10 o'clock maximum and 

 the 4 o'clock minimum, supposing it to be the same as in the plains of 

 India, might correspond to a difference in altitude of about 130 feet. How- 

 ever, observations made on the same day at no greater intervals than one 

 hour, and the shorter the interval the better, might be usefully compared 

 to show approximately the difference in level between the bottom and the 

 top of a mountain pass. The probable error for a difference of time of one 

 hour would not be greater than 20 feet in excess or defect according to the 

 circumstances. Such observations could be very conveniently made with a 

 good, large aneroid, which should be frequently compared with the mercu- 

 rial barometer, as aneroids are very liable to get out of order. 



Geography. 



The main objects of Geographical research are doubtless well understood 

 by the members of this Expedition, who have had so much practical ex- 

 perience in traversing unknown countries, and especially Mr. Nej r Elias, the 

 account of whose travels through China and Mongolia to the Russian 

 capital, in the Journal of the Royal Geographical Society, is so well known- 



