14 



KNOWLEDGE 



[Januaby 1, 1895. 



The motion at present is probably so slow that its exact 

 nature can hardly be determined — it is, in fact, in the 

 transition stage. 



The pair should be closely watched, and the measures of 

 different observers compared. The motion may not only 

 prove to be orbital, but the period may be much less than 

 is commonly imagined. Truly yours, 



W. H. S. IMONCK. 



To the Editor of Knowledge. 



Dear Sir, — In the recent issite of the British Astroiuimind 

 Association Journal, Vol. Y., No. 1, the section of Astro- 

 nomical Progress concludes an article on jo Lyr* with the 

 following thesis : — "A black stripe, then, overlaying a vivid 

 ray in the spectrum of jB Lyne or elsewhere, can only 

 be interpreted as a rever.sal. It demonstrates physical 

 changes in the atmosphere of a single star." 



This is obviously a conclusion of the first importance in 

 stellar spectroscopic interpretations ; and it is necessary 

 to examine the connection between it and its premisses. 

 For these the reader is referred to an article by Miss A. M. 

 Clerke in the June number of Knowledge, page 130, 

 and briefly the argument is contained in the following 

 propositions ; — 1. A combined spectrum from two close 

 stars (unaffected by the displacements of relative radial 

 motion) " is the simple summation of the rays." (" Not an 

 algebraical summation. There are no negative qualities. 

 The darkest ray cannot sink below the zero level of light.") 

 2. Therefore "vivid rays lose none of their light by 

 subtraction. 3. Therefore " dark lines can never be seen 

 projected upon bright ones emitted by a companion 

 luminary." 



In this argument the first proposition and its consequent, 

 the second, are undeniable ; but the third proposition, or 

 second consequent, goes too far, and does not militate 

 against an imitation of a double reversal by the super- 

 position of two spectra, one of which gives the hydrogen 

 lines broad and bright, the other narrow and dark. In 

 this case the narrow dark line will be seen dividing the 

 broad bright line, not by subtraction of the bright line's 

 intensity, but by less aildition to it. The added quantities 

 here are the intensities of the continuous spectrum of the 

 dark line star on the two sides of the dark line ; so that 

 the sum of light, on either side of it, is greater than at the 

 position of the dark line. 



It is true that this efl'ect of superposition is attended 

 with diminished contrast both between the bright line and 

 its dark division, and between the bright line and the 

 combined continuous spectrum ; but not necessarily to the 

 exclusion of either. 



Stonvhurst Observatory. ^Y alter Sidgreaves. 



MKCHANTCAL FUaHT. 

 To the Editor of Knowledge. 



Sir, — Referring to the December number of Knowledge, 

 Mr. Moy says : — " The two greatest obstacles in the 

 way of the accomplishment of mechanical flight have 

 been the balloon and the screw propeller." Mr. Moy 

 evidently does not believe in the screw ; he rather ridicules 

 " Mr. Maxim and Lord Kelvin ' for believing in this kind 

 of a propeller. 



Before commencing my experiments in aeronautics, I 

 consulted one of the old members of the Aeronautical 

 Society in regard to the efficiency of screw propellers, and 

 he assured me that he himself had made a great many 

 experime;its and had witnessed many that had been made 

 by others, including those of Mr. Moy, and in all cases it 



had been found that a screw propeller drew in air at the 

 centre and discharged it at the periphery, and that there 

 was a great deal of power wasted in this " fan-blower " 

 action — in fact, so much that the screw propeller was 

 absolutely of no value for aerial navigation. I then went 

 to France, and saw the apparatus which the French 

 Government used for testing the efliciency of screw 

 propellers working in air. It was very beautifully con- 

 structed, and enabled one to measure the power required 

 for driving the screw and also to determine with a great 

 degree of nicety the thrust of the screw. ]5ut there were 

 no means provided for allowing the screw to travel through 

 the air, and to ascertain the thrust while it was travelling. 

 Moreover, the screw propellers themselves — and there were 

 a dozen of them — were so badly made as to render the 

 experiments of no value. 



In regard to the early experiments in Englan<l with 

 screw propellers, I would say that a few semi-scientific 

 gentlemen, without any special engineering training and 

 with only a limited amount of money at their disposal, have 

 conducted a few experiments on a small scale. These 

 gentlemen have generally made their screws by loosely 

 covering some badly- formed and rickety wooden or wire 

 frames with a cotton or silk fabric. They have applied an 

 unknown quantity of power, having no means of ascertaining 

 the amount consumed or the thrust of the screw. Their 

 experiments have been made while the screw was rotating 

 in the same place, and not while it was advancing into new 

 air, the inertia of which had not been disturbed. As might 

 be expected, the results have been anything but satisfactory ; 

 nevertheless, these gentlemen have constructed certain 

 formula which they insist should be implicitly accepted by 

 those who have more recently conducted experiments 

 on a very much larger scale and with very much better 

 apparatus. 



Ill my early experiments with screw propellers, I pro- 

 vided myself with an apparatus, constructed in such a 

 manner that it would enable me to ascertain with a great 

 degree of nicety the exact power consumed in driving my 

 propellers, not only while stationary but also while 

 advancing at a high velocity into new air. It also enabled 

 me to accurately measure the screw thrust under all 

 conditions. In one of my first experiments with a well- 

 made screw I found, if I multiplied the pitch of the screw 

 in feet by the number of turns in a minute, by the thrust 

 in pounds, and divided the product by thirty-three thousand, 

 that it exactly corresponded witlr the readings of my 

 dynamometer. This indicatnl that there was no skin 

 friction. I then made a pair of blades without any twist 

 or angle, and exactly the same thickness and area as the 

 screw blades, and upon rotating tbese in the air at the 

 same velocity I found, notwithstanding that my dynamo- 

 meter was exceedingly sensitive, that the power required 

 was so small as not to move the pointer. This proval that 

 the skin friction was so small a factor as not to be con- 

 sidered. I'pon trying the same experiments with a screw 

 made exactly like those I had seen in France, I found, 

 when its efficiency was computed on exactly the same 

 basis, that it did not foot up to more than one-third the 

 readings of the dynamometer, and I presume that this 

 screw was quite as efficient as any that have ever been 

 made or experimented with by Mr. Moy or his colleagues, 

 and which have formed the basis of their dogmatic asser- 

 tions. 



In regard to the wasteful effects of the "fan-blower" 

 action before referred to, I found, with a well-made screw, 

 that the air instead of being drawn in at the centre and 

 discharged at the periphery, as these gentlemen had 

 supposed, was in reality — when not advancing into new 



