THE CONQUEST OF THE AIR. 357 



and this part of the machine slows down. In addition to the 

 warping:, rudders are fixed to the elevators or the tails. 



La^teral stability may be increased by the agilitv of the pilot 

 and by lateral curtains, as in the Voisin biplane.' Automobile 

 stability IS not yet employed, although a number of patents have 

 been granted. The brothers Wright employ in one of their 

 patents compressed air, forced against planes. Gvroscopes have 

 also been thought of. 



Of great importance is the proper working of the motors and 

 the greatest care must be exercised in their construction They 

 must be very light (i| to 3 kgs. per H.P.) and very reliable. The 

 stopping of the motor would be of little importance if thereby the 

 centre of wind pressure were not changed. It is this which the 

 pilot has to watch and to counterbalance. 



A great deal depends also on the pron-Ilors. Thev should be 

 as sm.ooth as possible, to reduce friction to a minimum, and the 

 pitch of the blades should be such that no part is a drag on 

 another. Large slow speed propellers vield better results than 

 small high speed ones. 



SPEKBOOM— Mr. Ernest E. Galpin, writing to Mr. 

 J. Burtt-Davy apropos of the latter's observations regarding the 

 occurrence of Spekboom in the Transvaal Province (vide page 264 

 of this volume) remarks that it occurs well inland of both the Ama- 

 tolas and the Kologha Range— the next higher plateau. He 

 goes on to say : " It grows on the banks of the Zwart Kei River, 

 close to its junction with the White Kei, in the Cathcart District' 

 and it is quite likely that it crosses the river and is on the Queens- 

 town side. The country is verv wild, rugged and inaccessible 

 in the neighbourhood of the junction of the Kei Rivers, and quite 

 a Karroo flora follows the deeply eroded valley of the Kei right 

 into the Queenstown District, with such plants as Rhigozum 

 tnchotomum, Capparis alhitrunca, Schotia speciosa, Azima tetra- 

 cantha, ^c. 



HALLEY'S COMET. — During April and the earlv part 

 of May this comet was still being closely observed bv 

 Prof. Barnard, its magnitude in April being 15. ' Mr. F. Slocum 

 a few weeks ago obtained a good photograph with a 2ft. reflector 

 after one hour's exposure, in which it showed brighter than on 

 September 11, 1909, although its solar distance was 110,000,000 

 niiles more than on the date mentioned, a proof of the persistence 

 of the physical brightening at nerihelion. 



