96 



NATURE 



[Nov. 27, 1879 



When the fork vibrates, this image is drawn out into a band of 

 light. When the mirror commences to revolve, this band breaks 

 up into a number of moving images of the mirror ; and when, 

 finally, the mirror makes as many turns as the fork makes vibra- 

 tions, these images are reduced to one which is stationary. This 

 is also the case when the number of turns is a sub-multiple. 

 When it is a multiple, or a simple ratio, the only difference is 

 that there will be more than one image. 



Hence, to make the mirror execute a given number of turns, 

 all that is necessary is to pull the cord attached to the valve, to 

 the right or left, till the images of the revolving mirror come 

 to rest. 



The electric fork made about 128 vibrations per second. No 

 dependence was placed upon this rate, however, but at each set 

 of observations it is compared with a standard Ut 3 fork, the tem- 

 pera ure being noted at the same time. In making the comparison 

 the beats were counted for 60 seconds. 



It is interesting to note that the electric fork, as long as it 

 remained untouched and at the same temperature, did not 

 change its rate more than O'OI or o'02 vibration per second. 

 Fig. 6 represents the table at which the observer sits. The 

 light from the heliostat passes through the slit at s, goes to the 

 revolving mirror, etc., and on its return forms an image of the 

 slit at D, which is observed through the eyepiece. E represents 

 the electric fork, bearing the steel mirror, II ; K, the standard 

 fork on its resonator. 



The lens was made by Alvan Clark. It was 8 inches in dia- 

 meter, focal length 1 50 feet, not achromatic. It was mounted in 

 a wooden frame, placed on a support moving on a slide about 

 1 6 feet long, placed about 80 feet from the building. As the 

 diameter of the lens wis so small in comparison with its focal 

 length, its want of achromatism was inappreciable. For the 

 same reason the effect of "parallax" was too small to be 

 noticed. 



The stationary mirror was one of those used in taking 

 photographs of the transit of Venus. It was about 7 inches 

 in diameter, mounted in a brass frame capable of adjustment in 

 a vertical and a horizontal plane by screw motion. Being pris- 

 matic, it had to be silvered on the front surface. To facilitate 

 adjustment, a small telescope furnished with cross hairs was 

 attached to the mirror by a univer.-al joint. The heavy frame 

 was mounted on a brick pier, and the whole surrounded by a 

 wooden case, to protect it from the sun. 

 The adjustment was effected as follows : — 

 A theodolite was placed at about 100 feet in front of the 

 mirror, and the latter was moved about, till the observer at the 

 theodolite saw the image of his glass reflected in the centre of 

 the mirror. Then the telescope attached to the mirror was 

 pointed at a mark on a piece of cardboard attached to the 

 theodolite. Thus, the axis of the telescope was placed at right 

 angles to the surface of the mirror. The theodolite was then 

 moved to 1,000 feet, and, if found necessary, the adjustment 

 repeated. Then the mirror was moved till its telescope pointed 

 at the hole in the shutter of the building. The adjustment was 

 completed by moving the mirror by signals, till the observer, 

 looking through the hole in the shutter through a good spyglass, 

 saw the image of the glass reflected centrally in the mirror. 



Notwithstanding the wooden case about the pier the mirror 

 would change its position between morning and evening, so that 

 the last adjustment had to be repeated before every series of 

 experiments. 



(To be contimted.) 



ON THE MOUNTAINS OF THE NORTHERN 



AND WESTERN FRONTIER OF INDIA 1 

 ■""THERE are certain moot questions relating to the mountains 

 of the north-west frontier of India upon which it appears 

 desirable to elicit the opinion of geographers. On this occasion 

 I propose to discuss the western limits of the Himalaya ; the 

 northern and southern Hants of the Hindu Kush ; the parallelism 

 and lateral communications of the ranges between the Hindu 

 Kush and the Aralo-Caspian plain and of other parts of the 

 north-west frontier; and the limits of the Iranian group of 

 highlands, at its junction with the Tibeto-Himalayan and 

 Pamir groups. Finally the proper route of a railway to 

 India between Mesopotamia and the Indus is indicated along a 

 remarkable line of elevated valleys parallel to the coast. 



1 Paper read at the Sheffield meeting of the British Association by 

 Trelawney WVSaunders. 



The Himalaya ranges form a part of the great girdle of 

 mountains which continuously encircle the central porlion of 

 the Asiatic Continent, and include the Chinese colonial depend- 

 encies of Hi, Mongolia, Kokonor, and Tibet. 



This vast mountain girdle is naturally grouped into four parts 

 corresponding with the outlets of its exterior drainage according 

 to their connection with the Arctic, Pacific, and Indian Oceans, 

 and the Aralo-Caspian Seas respectively. These four divisions of 

 the great mountain girdle have been named from their chief 

 features, (1) the Tibeto-Himalayan, (2) the Yunling, (3) the 

 Altaic, and (4) the Pamir systems. The interlacing or over- 

 lapping of these systems at their junctions is not always easy to 

 make out, and presents occasional difficulties like other systems 

 of classification, no matter what the subject may be. 



The question of the western termination of the Himalaya 

 relates to two divisions of the great girdle, namely, The Tibeto- 

 Himalayan and the Pamir, to which must be added another 

 group lying outside those two, but impinging on them at its 

 north-east corner. This group is formed by the Iranian high- 

 lands, a compact quadrilateral mass bounded by the lowlands of 

 the Indus, the Arabian Sea and the Persian Gulf, the lowland 

 of the Tigris and Euphrates, and the Aralo-Caspian plain. The 

 only questions which can well arise with regard to the boundaries 

 of the Iranian mountain system, relate to its nonh-west and 

 north-east angles, where it unites with the Tauric system on the 

 one hand and the Himalaya and Pamir on the other. It is with 

 the latter only that we propose to deal now. 



My Tibeto-Himalayan system was introduced in 1870 in "A 

 Sketch of the Mountains and River Basins of India in two Maps, 

 with Explanatory Memoirs." It was further developed in the 

 Geographical Magazine, for July, 1877 ; and I am indebted to 

 the distinguished chairman of this section for an appreciative 

 account of it in two editions of his "Memoirs of the Indian 

 Surveys in 1871 and 1877." It resolves the leading features of 

 the vast mass of which it treats into four great chains with their 

 outer slopes and intermediate valleys or plateaus. I am obliged 

 to allude briefly to this bygone work, for the purpose of forming a 

 logical basis for »he argument which follows. 



The northern and southern Himalaya are two of these great 

 chains. The Karakorum-Gangri and the Kuenlun are theother two. 

 The Southern Himalaya rises from the great plain of India, and.its 

 culminating summit is distinguished by an extraordinary chain 

 of snowy peaks throughout the whole extent which is claimed 

 for it. The catenary and close succes-ion of the e snowy peaks 

 cannot be denied, for they have been fixed in position and 

 altitude by the indubitable observations of the great Trigono- 

 metrical Survey of India. Nor can the existence be disputed of 

 the line of valleys which forms the northern base of this snowy 

 range and distinctly separates it from the Northern Himalaya. 



Yet an antiquated theory conceived before the exi-tence of this 

 snowy range was demonstrable, is still held to be possibly tenable 

 by the authors of the recently published manual of the Geology 

 of India, although they do not condescend to any reason for their 

 conclusion. 



Now this is not merely a matter of dispute between geologists 

 and geographers, but it is one of the greatest practical importance 

 w ith reference particularly to the potent question of lateral com- 

 munications about which much has been said lately in reference 

 to the Iranian system in Afghanistan. In the successive valleys 

 following one after the other in the same line, each of which I 

 have specifically named in the Geographical Magazine, there is 

 indisputable evidence of the separation of the two ranges, and 

 of that lateral communication which is an ordinary feature of 

 mountain systems, rather than otherwise. 



The Northern Himalaya has its southern base in these valleys, 

 while its northern base is found in the extended trough along 

 which flows the upper courses of the great rivers Indus, Sutlej, 

 and Sanpu. 



It is usually said that the Himalaya extends up to the gorge of 

 the Indus on the west and to the gorge of the Sanpu on the east, 

 and this is the extent assigned to the Himalaya by the authors of 

 the "Manual of the Geology of India." But this restriction falls 

 short of the limits which we have already assigned to the Tibeto- 

 Himalayan system, on the basis of the natural oceanic watersheds. 

 It also falls short of the extension attributed to the Himalaya 

 on the west by observers and geographers of celebrity ; and we 

 shall endeavour to prove that it falls short, on the west at least, 

 of the plain and simple application of the same conditions as 

 those on which the Himalaya is allowed to extend up to the 

 gorge of the Indus. 



