Novi 



1901.] 



KNOWLEDGE. 



259 



fall of ono inch of raiu means that over every aero 

 of gi-ound a weight of one hundred tons of water has 

 fallen, or 60.000 tons to the square mile. Put in 

 another wav, this downpoiu- over such a welt-known area 

 as the Thiiines Vallej' me.vns that 53,000.000.000 gallons 

 of water have been precipitated from the atmosphere. 

 It has been calculated that the condensation of one 

 gallon of rain gives out enough latent heat to melt 

 75 lbs. of ice. or to melt 45 lbs. of cast iron. From 

 these figures the mathematically inclined may work out 

 for themselves the amount of heat set free in some 

 ti-opical downpoiu- when the rain instead of being an 

 inch in depth, is seven or eight. That this liberated 

 heat has givat effects on the temperature and move- 

 ments of the air goes without saying, but this part of 

 the subject must not here be further pursued. It is 

 now sufficient to say that just in the same way that 

 condensing rain gives out heat, so do hoar-frost and 

 dew. and a recognition of this fact has resulted in the 

 suggestion of a rule for foretelling the occurrence of 

 hoar-frost. 



The success of this prognostic depends on the fact 

 of there being an intimate relation between the 

 deposition of hoai--frost and the temperature of the 

 dew point, as it is termed. Reference was made above 

 to the circumstance that the amount of moistiu-e a given 

 body of air can hold depends on its temperature. Thus 

 at a temperature of 52 deg. a cubic foot of air is capable 

 of giving accommodation to 4'39 gi-aius of vapoui", but 

 at 32 deg. there is room only for 218 gi-ains. If, there- 

 fore, a bod\' of air at the former temperatui-c is 

 suddenly cooled, its capacity for moisture is con-espond- 

 ingly reduced and some of the aqueous vapom' spills 

 over, as it were, or is condensed. From this it will be 

 seen that there is a critical temperature below which 

 any vapoui'-laden air cannot be reduced without some 

 of the moisture spilling over; this critical temperature 

 being called the dew point. A glass of cold water (to 

 repeat an illustration), when brought into a wai'm room, 

 reduces the temperature of the air in contact with it 

 to the dew point, so that drops of moisture fomi on 

 the outside of the glass. Instiniments that give the 

 temperature to which the air is thus reduced are called 

 hygi-ometers, and during frosty weather, as - ah'eady 

 suggcstetl. a knowledge of this dew point may become 

 exceedingly useful. 



If. for instance, in the evening, the hygrometer shows 

 the dew point to be above 32 deg.. in the majority of 

 cases there will be no hoar-frost that night. On the 

 other hand, if the dew point is below 32 deg.. and if 

 there is a moderate amount of moisture in the air. plenty 

 of hoar-frost may be expected. From what has 

 previously been said it will be seen that this prognostic 

 is capable of a vei7 simple explanation. Latent 

 heat is set free when condensation of moisture takes 

 place, so that when the dew point is above 32 deg., any 

 deposition of moisture results in a little warmth 

 appearing, which is often quite sufficient to ward off 

 hoar-frost. On the other hand, with a dew point below 

 32 deg., these hidden stores of heat are not sufficient 

 to hold in check the advance of the icy spicules. 



But probably the most interesting fact in connection 

 with hoar-frost is its giowth, it being no uncommon 

 thing to see favourable surfaces literally submerged in 

 a frosty mantle. Hoar-frost, moreover, is better suited 

 by an atmosphere where moisture is plentiful than 

 when it is not so abundant, these latter conditions being 

 more favourable for the birth of dew-drops. Another 

 consideration is that moisture mav be reduced below 



the freezing point without congealing. It is a common 

 experiment thus to treat moisture, although the sliglitest 

 shaking of this cooled liquid is enough to change it 

 promptly to a solid form. Similarly as regards the 

 moi.stin'c in the atmosphere, there are excellent reasons 

 for supposing that in certain circumstances the vapour 

 may bo a degree or two below the freezing point without 

 actually solidifying, and is only waiting the touch that 

 will turn it into a feathery frond of ice. Supposing, 

 then, that moist air in this condition is gently wafted 

 against a bush, a fence, or a blade of grass, the shock, 

 though slight, is quite enough to work a magical trans- 

 formation. The greater part of these frost effects arc 

 thus prepared in the air. and as each body of chilled 

 vapour floats against an object having already' upon It 

 a covering of frost, it is. as it were, roused by the shock, 

 and awakening, promptly adds its load of frozen crystals 

 to the growing picture. 



THE WATER OF THE DEAD SEA. 



By C. Ai.Niwoi;Tii Mitcuei.i., u.a., k.i.c. 



Therk arc few more interesting sites in the world than 

 tho largo lake which receives the waters of the Jordan. 

 Its historic interest, tho marvellous legends which were 

 accepted and passed on by mediaival travellers, its 

 desolate position, and the remarkable character of its 

 water have all helped to make it famous. 



In the Bible it is called T/ie Suit Sea and Tlie Sea of 

 tlic Plain ; whilst among the Greeks and Romans it 

 was known as Lake AxijhaU'dKx, from the bitumen found 

 floating on its siu'face. Its more modern name of Mare 

 Mart II II HI or T/ie Dead Sea is due to the absence of life 

 in its waters and the scanty vegetation on its shores. 

 Among the Arabs it is still known as Balieiret Lut or 

 The Sea of Lot. 



In shape it is an irregular oblong figure somewhat 

 resembling a bow, the general direction of which is from 

 north to south. On the east and west it is enclosed 

 by ranges of mountains, which in places rise in sheer 

 precipices from the water to a height of over 2,000 feet. 

 Josephus (Bell. Jud. IV., Cli. 8) gave its dimensions as 

 72 miles long by IS miles wide, but explorations during 

 tho nineteenth century have shown that it is now only 

 46 miles in lengt.h by about 8 miles in width. Some 

 coiitriiction of tlie area has undoubtedly occurred, but 

 even allowing for this, the earlier estimate must have 

 Ijeon too great. The surface of the water is more than 

 1.300 feet lx;low the level of the sea. The bottom is 

 rocky and uneven, tho northern pait being very deep, 

 wliilst the southern section below the peninsula of Lisan 

 seldom exceeds ten feet in depth. 



The lake thus lies in a deep cavity, and being exposed 

 to the burning rays of the sun. evaporation proc'ceds 

 so rapidly as to more than counterbalance the influx of 

 the Jordan. Incnistatious of salts mark the higher 

 levels reached by the water when the river is flooded. 



To tho south-west lies an extensive range of hills of 

 rock salt, whilst sulphui- mines aie found on the west 

 coast, and sulphur springs all along the shore. The 

 origin of the asphalt which floats in masses on the water 

 is attributed to the '' slime pits " in tho vale of Siddim 

 (Gen. XIV., 10). This asphalt is driven by tho wind 

 to the wcsteni and southern shores, where it is still 

 collected by the Arabs and sold for pitch and for 

 medicinal pui-poses. According to tradition, it was used 

 bv till- juifioiit Egvptians in embalming their dead. 



