360 APPENDIX A. 



being about 10 metres, a sma,!! correction of + 0-9 mm. has 

 been made in order to obtain the corrected reading adopted in 

 the following table for the ' Mogador barometer.' 



As most of our observations at stations in South Marocco 

 were necessarily made either early in the morning or late in the 

 evening, while those at Mogador were registered at 10 a.m. and 

 4: P.M., the pressure at the latter place corresponding to the 

 hour of each of our observations has been found by intercalation. 

 There is of course an obvious possibility of error here ; but, 

 except on a few occasions, when the changes of pressure were 

 considerable and rapid, the amount is probably trifling. 



It is familiar to all who have given attention to this sub- 

 ject, that one of the chief causes of error in the results obtained 

 from barometric observations for altitude arises from the im- 

 possibility, in the present state of knowledge, of obtaining with 

 tolerable accuracy the temperature of the stratum of air lying 

 between the lower and the higher stations. This is especially true 

 in climates such as that of South Marocco, where the sky is 

 commonly clear, and the air relatively dry. The cooling of the 

 surface at night, and the heating in the sunshine by day, have 

 an effect on the layer of air in contact with that surface, and 

 still more on the traveller's thermometer, which at the best is 

 imperfectly protected from radiation, out of all proportion to 

 the actual cooling or heating effect on the air not in immediate 

 proximity to the soil. As far as circumstances permitted, it 

 was sought to take observations about an hour after sunrise and 

 very soon after sunset, so as to diminish to the utmost this 

 source of error. 



It remains true, in the writer's opinion, that when all these 

 sources of error in the determination of height^ by means of 

 the barometer have been put together, there remains one sur- 

 passing all the others in amount which altogether escapes our 

 means of correction. The formulae employed for the reduction 

 of observations to numerical results are, and must be, based on 

 the assumption that a condition of equilibrium between the 

 forces acting on the instruments at each station has been 

 attained ; whereas the utmost that can be asserted is that there 

 is a continual tendency towards such equilibrium, requiring a 

 variable time to effect it. But before equilibrium can be 

 attained new changes occur, and the process of adjustment 



