Sept. i 3) 1883] 



NATURE 



479 



stock notion? of how the monsoons are caused in India are so 

 prevalent. The explanations of many of the text-books are in fact 

 a libel on the intelligence of both teacher and pupil. 



It is usually sail that the air is heated over the land ex- 

 pands, and rises (presumably in a courant ascendant). The 

 air from the sea then rnshe < in to supply the tendency to vacuum, 

 and this constitutes the monsoon. 



The true state of the ease is, however, quite different. The 

 way in which the air is removed from North India is not by 

 ascending, but by lateral currents which constitute the "hot 

 winds." The vertical expansion by which a larger portion of 

 1 he whole atmosphere is lifted above the level of the hill-stations 

 would indeed rather tend to raise than lower the isobaric planes 

 towards the north, and as no true "courant ascendant" can exi-t 

 until the air is rendered moist by the monsoon rains, the lateral 

 winds are the only means by which the isobaric planes are caused 

 to slope northwards prior to their arrival. 



The annual variation of pressure at the level of Leh, which is 

 11,53s feet above sea-level, shows us that at a still higher eleva- 

 tion the phases of annual barometric variation are exactly contrary 

 to those which occur on the plains, the minimum occurring in 

 mid-winter, and the maximum in midsummer. 



It is further shown by Prof. Hill that the peculiar double 

 oscillation at the bill-stations, which in correspondence with 

 .heir position is intermediate in character to those in the two 

 extreme cases, is due to exactly the same causes as the single 

 oscillation on the plains — a fact which will prove of much utility 

 in further research on this complicated question. 



The winds for the most part correspond to the barcnetric 

 variations. The constant southwesterly direction of the wind 

 at the elevation of the hill-stations is, the result of two indepen- 

 dent circumstances, viz. (1) the small depth of the winter (north- 

 cat i monsoon, above which the south-west anti-monsoon blows, 

 and (2) the great height to which the summer (south-west) mon- 

 soon reaches. Above the latter monsoon it is not known how 

 the wind blows, but in accordance with cyclonic laws it sho.ild 

 be north-west. Perhaps future research will verify this inference. 



The discussion of the humidity observations, leads to results 

 which corroborate some previously obtained from somewhat 

 meagre data by General Strachey. On the assumption that 

 Hann's empirical formula with the value of the constant as given 

 by Hill is correct, viz. — 



log/ = log/"-—*—, 1 

 * r ° 23058' 



it is found that " at an elevation of 23,000 feet, or about the 

 average height of the snowy peaks, the quantity of vapour in 

 the air is only one-tenth of that at sea-level. The extreme dry- 

 ness of Thibet and Ladakh is thus easily accounted for." 



The relative humidity depending on the temperature, obeys 

 quite different laws, and undergoes variations very similar to 

 those in the amount of cloud. 



The average height at which cloud would be formed in the 

 rainy season, is calculated by Prof. Hill to be about 4000 feet, 

 and it is interesting to note that this elevation agrees with that 

 of the zone on which the greatest amount of rain falls in the 

 Himalaya, the exact height of which is found to be 4240 feet 

 above sea-level. Above this height the rainfall decreases 

 rapidly owing to the exhaustion of vapour, but in the case of the 

 Himalaya this decrease is rendered more prominent ow ing to the 

 outer ranges cutting off the supply of vapour to those more in 

 the interior by promoting abnormal precipitation in their own 

 vicinity. E. Douglas Archibald 



MULTIPLEX CAMERA BACK 



"THE great advance in tourist photography by reason of the 

 production of the more sensitive and rapid gelatine dry 

 I lates now used in such large numbers has led to continued 

 improvements in the construction of portable photographic 

 apparatus. 



Considerable difficulty has always been experienced in carrying 

 a sufficient supply of sensitive plates for a day's tour. 



To meet this want not only are large numbers of double backs 

 carried but the changing box has also been devised. The latter 

 consists of a cabinet arranged to carry twelve sensitive plates and 

 a specially constructed dark back for the camera. When a 

 plate is required to be changed, the dark back is attached to the 



1 Where / P are the vapour tensions at the given elevation and sea level 

 respectively, amd h is the height in feet. 



changing cabinet, and by the action of springs and shutters a 

 sensitive plate is transferred from the cabinet to the dark back, 

 which is then removed and exposed in the camera as desired. 

 The changing box is complicated and expensive, besides adding 

 another piece of apparatus to the tourist's lujgage. The greatest 

 difficulty, however, arises from the very merits of the gelatine 

 plates themselves. 



They are so sensitive that the utmost care is required to keep 

 every trace of light from the plate, and douMe backs that appear 

 perfect to the eye, yet by the action of the sensitive plates them- 

 selves are found to be imperfect. It is obvious that the multi- 

 plication of double backs and the shutters forming part of them, 

 adds to the liability of access of light and consequent fogging of 

 plates. With the use of a changing box the same trouble is 

 experienced, with occasionally further difficulties, caused by 

 variations in thickness or sizes of sensitive plates, the latter 

 sometimes refusing to pass from the changing box to the back or 

 vice versa, very often causing loss of time, temper, and plate as 

 well. 



We give illustration and description of an improved appar- 

 atus that, by its simplicity of action, appears to obviate the 



difficulties before mentioned, and to possess merits of its own 

 that will insure the success desired by the inventors. The ap- 

 paratus combines in one cabinet the dark back and the changing 

 box, and is the invention of Messrs. J. H. Hare and H. J. 

 Dale. 



The woodcut (which shows part of the outer cabinet cut away 

 to give a view of the interior construction) will immediately 

 explain its action. 



The cabinet or multiplex back is made large enough to contain 

 thirteen plates in two tiers, the lower tier containing seven and 

 the upper tier six plates. The plates are secured in holders or 

 carriers, with a thin metal back to each to prevent the light 

 passing through the plate which may be exposed to those 

 behind it. In the front of the cabinet is the usual sliding shutter, 

 which draws up half way for exposure of the front plate of the 

 lower tier. 



At the back of the cabinet is a shutter which can be entirely 

 removed when required to refill the back with plates. In the 

 front shutter a small window of non-actinic glass is provided, 

 through which the number of the sensitive plate ready to be 

 exposed can be seen. In the back shutter two quick-running 

 three-thread screws are provided, the lower one to bring 

 the plates of the lower tier up to focus, and the 



