CORRESPONDENCE. 



THE FOURTH DIMENSION. 



To the Editors of " Knowledge." 



Sirs, — I am glad to take this opportunity of replying to 

 Mr. F. W. Henkel's letter on the above, because he appears 

 to have a definite argument to offer on the subject. This 

 argument, if I mistake not, is that experience only gives us 

 solid bodies, which we, somewhat arbitrarily, divide up into 

 three dimensions, i.e., that the concept of a three-dimensional 

 body is not, as the term implies, a synthetic product, but is 

 given immediately by experience ; whilst the concept of a 

 one-dimensional object (i.e., a line) is not an element out of 

 which the former concept is built, but is the product of 

 analysis. Consequently, says Mr. Henkel, any argument 

 based on the real existence of one-dimensional objects is 

 fallacious, because such objects do not in fact, really exist. 

 But .... in fact, I have a string of buts. First of all, I 

 would ask Mr. Henkel what he understands by existence. 

 What criterion of real existence can he offer which denies 

 reality to that which exists in mind ? But to leave aside the 

 question of epistemological idealism, surely Mr. Henkel will 

 not deny the existence of one- and two-dimensional objects as 

 aspects of three-dimensional objects ; and if this be so, 

 there is nothing in his argument to prevent a belief 

 that three-dimensional objects are merely aspects of four- 

 dimensional objects, and so on ad infinitum. And that 

 is exactly what I have suggested. Moreover, are all the 



objects of experience three-dimensional ? I think not. Who 

 has ever seen a three-dimensional body ? I have not. What 

 I only see, and what, I think, Mr. Henkel only sees, are flat, 

 coloured surfaces (i.e., two-dimensional). We simply infer 

 them to be three-dimensional, because that is the idea we 

 gain through our sensations of touch and motion. Indeed, by 

 combining our sensations of touch and motion with our sense 

 of duration, we may be said to experience four-dimensional 

 objects — my experience, for example, tells me that the paper 

 on which I write has length, breadth and thickness, and also 

 duration in time, i.e., it has four dimensions. 



It is best, I think, "to look at the subject from the point of 

 view of motion. Experience tells me that I can move (to 

 some extent at least) in an indefinite number of directions in 

 space, but these may all be resolved into three (but not less) 

 directions. The reality of the fourth dimension merely 

 implies the possibility of movement in a new direction irresolv- 

 able into these three directions. As I have suggested above, 

 time may be this direction : its apparent difference from the 

 spatial directions being due to the fact that we are forced 

 along time with no option of any other movement so long as 

 that direction is concerned, and that we seem to have practi- 

 cally no power to see in the direction of time (unless, perhaps, 

 memory is a sort of time-sight, and prevision be a fact). 



The Polytechnic, 

 Regent Street, London, W. 



H. S. REDGROVE. 



SOLAR DISTURBANCES DURING JANUARY, 1913. 

 By FRANK C. DENNETT. 



January was poorly favoured with suitable weather for the 

 solar observer. Seven days (4, 10, 11, 19,23, 29 and 30) were 

 missed entirely. On eleven days (5, 6, 8, 9, 12, 13, 20, 21, 22, 

 24 and 28) the disc appeared free from disturbance, bright or 

 dark. The longitude of the central meridian at noon, on 

 January 1st, was 6° 12'. 



No. 26. — -A group belonging to December, which continued 

 visible until January 3rd, and so appeared on the chart last 

 month. 



No. 1. — A group of two spotlets and two pores first seen on 

 the 14th, in a faculic disturbance approaching the western 

 limb. Next day only one pore was seen within a ring of 

 faculae, very near the limb. The group appeared to be 

 50,000 miles in length. 



No. 2. — On the 16th, an elongated faculic cloud well round 

 the south-eastern limb was seen to contain a spotlet with a 

 gray companion closely north of it, and a minute pore a little 

 ahead. It remained visible as a hazy pore on the 17th. 



No. 3. — On the 25th, two spotlets were visible, and also on 



the 26th, but the distance between them had increased to 

 37,000 miles. On the 27th two pores were seen but not nearly 

 so far apart, and there were traces of a minute point nearly in 

 the place of the rear spotlet, but none were observed after. 



The faculic display was also very little. On the 7th some 

 was visible within the eastern limb, probably the remains of 

 groups 24 and 24a, which is shown on the chart, longitude 

 206°-220°, as seen again on the 17th and 18th. On the later 

 date less conspicuous faculae were recorded, but not measured, 

 within the north-east and south-west limbs. On the 26th 

 there was a bright knot at longitude 334°, 18° North latitude. 

 On the 31st some faculic spots appeared in 30° North latitude 

 between longitudes 41° and 51°, a little within the north- 

 western limb, and doubtless were the remains of the disturb- 

 ance which produced No. 26. 



The helpers whose observations have contributed to the 

 preparation of our chart are Messrs. J. McHarg, A. A. Buss, 

 E. E. Peacock, W. H. Izzard, and the writer. 



Erratum. — On p. 69, line 9 below the first chart, in the 

 second column, for 228, read 288. 



DAY OF JANUARY. 



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