46 THE VOYAGE OF ELMS. CHALLENGER. 



the problem of the rate of diminution of temperature with height. This consideration 

 has unfortunately been often lost sight of, particularly in framing tables of corrections 

 for height intended for different climates and seasons. 



In differentiating for stations at which observations were not made for the whole 

 of the fifteen years ending with 1884, in order to bring their means to the means of 

 these years, the same methods were adopted as those used in preparing the monthly 

 means of atmospheric pressure. Very special care was taken to differentiate coast 

 stations only with coast stations, and inland stations with inland stations. Also when, 

 in differentiating, the observations of only a few years were available, the geographical 

 distributions of abnormally high or abnormally low monthly temperatures during these 

 years were carefully noted in their bearings on the monthly means being worked out. 



Wind. — The observations of wind are given in Tables VII. and VIII. In all cases 

 where possible, the mean direction of the wind has been worked out in the form given 

 in Table VII. CUmatologically, the most satisfactory way of presenting this most 

 important element of climate is by giving the mean number of days each month which 

 each wind, N., N.E., E., etc., prevails. If only the mean direction is given, as is done 

 in Table VIII. , the variability of this important factor of climate from the prevailing 

 direction is absolutely neglected, and the climatic value of the record seriously 

 lowered. 



In this discussion no account has been taken of the force or velocity of the wind, 

 such observations being stdl too meagre and too crude for any satisfactory use being 

 made of them. 



It has not been possible, owing to the want of the observations, to give for many 

 regions the same weight as regards time to the means of the winds, as to the means of 

 pressure and temperature. This has, however, been done as respects the United States, 

 the North Atlantic, and a large portion of the Europeo-Asiatic continent, where these 

 three elements of climate are substantially synchronous, and where, therefore, their 

 relations can be more closely compared. So far, however, as affects the mean direction 

 of the wind, it soon appeared in the course of the discussion that a shorter term of years 

 is required to give a close approximation to the true means, than in the case of the 

 pressure or the temperature. Hence an attempt has been made, in those regions where 

 the observations are not obtainable for the whole period of the fifteen years, to collect 

 the ' averages for as long terms of years as possible. The hours of observation from 

 which the means have been calculated, when known, are stated ; and where a selection 

 of hours could be made, those hours were chosen which appear to give the best daily 

 mean in view of sea and land breezes. Wherever it could be attempted, means deduced 

 from hourly observations have been given, which alone really inform us as to the mean 

 daily direction of the wind. 



