Ii6 



SCIENTIFIC NEWS. 



[July 1st, 1887. 



useful little animal. Dr. H. J. Wharton, in his " Notes on the 

 Flora of Hampstead and Caen Wood," points out that the original 

 name of this locality is Ken Wood, and that Kentish Town would 

 be more correct if named Ken-ditch Town. Mr. A. Hammond 

 has studied the " Homologies of certain parts of Insects," and 

 remarks — that some persons are apt to forget that the microscope 

 is but a means to an end. Mr. V. A. Latham continues his in- 

 structions on injections used in the preparation of microscopic 

 specimens. At a meeting of the " County of Middlese.x 

 Natural History Society," Mr. W. Mattieu Williams remarked 

 suggestively that the ocelli of certain insects might serve for the 

 appreciation of that vast number of waves which are intermediate 

 between those of light and of sound, and which may furnish 

 these creatures with sense-images of which we have no con- 

 ception. 



Persons having the necessary leisure would do well to join the 

 "Postal Microscopical Society." 



CORRESPONDENCE. 



The Editor does not ho' d himself responsible for opinions expressed by 

 his correspondents, nor can he take notice of anonyjiioiis communica- 

 tions. All letters must be accompanied by the name and address of 

 the writer, not necessarily for publication, but as a guarantee of good 

 faith. 



THE FORMATION OF DEW. 

 To the Editor of Scientific News. 



Sir, — In your May number expression has been given to 

 approval of Mr. J. Aitken's views on the formation of dew by ex- 

 halation from the ground below, in contra-distinction to the 

 views of Wells on its formation from the air above. 



It may be mentioned that Mr. Aitken may not yet be entitled 

 to have everything explained, as it would not be so easy 

 to account for the heavy dews met with on board ship at 

 sea by his method. These are seen to occur in clear nights, 

 often very early, and to be deposited profusely on the decks, 

 which had already been dried and warmed by the previous day's 

 sunshine and wind. The moisture of the air at sea would appear 

 in our seas not to be derived from the sea direct, but from the 

 clouds above, and the atmosphere becomes charged with aqueous 

 vapour in the daytime by the sun's heat dissipating the visible 

 vesicular water of the clouds floating about. In the cabins and 

 saloons of iro>i ships of war, in their early construction and fur- 

 nishing, it was found that the walls inside were constantly damp 

 and streaming with moisture, to the extent of injury to health and 

 comfort. The disability was after a time quite rectified by lining 

 them with a non-conducting material, which prevented the access 

 of the warm 7?ioist air to the cold bare iron bulkheads. 



This insanitary dew was solely derived from the air permeating 

 the interior of the ship, and not from the materials of construc- 

 tion, and it would be charged also with the aqueous vapour 

 exhaled from the men on board in addition. — I remain, sir, yours 

 faithfully, Ros-Dew. 



To the Editor of Scientific News. 



Sir, — I have been pleased to see the letter addressed to you 

 by " Ros-Dew," as, although I cannot but differ from him, I am 

 glad to have the subject of my article freely discussed. I have 

 communicated the contents of this letter to my friend, Mr. Aitken, 

 of Darroch, and I have pleasure in stating that we agree upon 

 the answer which I now give. 



The difficulty expressed by your correspondent has been ob- 

 served by many others. There is, however, no real difficulty in 

 the matter. If the surface of the earth were covered with sheet- 

 iron or any substance which gave off no moisture, and if this 

 iron was not in good heat-communication with the earth, then it 

 might get cooled at night to a temperature low enough to cause 

 some of the moisture in the air to condense upon it, just in the 

 same way as the vapour is condensed on the plates inside a 

 ship in which a number of people are living, and giving off a 

 great quantity of vapour. The same thing may be seen inside 

 any large and crowded room in winter when the walls are cold. 

 Towards the end oi the meeting the walls will often be seen to 

 be running with moisture. So it is outside ; there is always 

 some moisture in the air, and if the surfaces of bodies get cooled 

 below the dew-point, they will have moisture deposited on them, 

 even though no vapour is rising at the time from the ground. 

 There are certain natural conditions when this maybe the case, 



as on board ship, when the temperature of the air is below the 

 dew-point, and not giving off vapour. 



These, however, are not the conditions which exist in nature 

 generally. The earth is not cased in iron or other damp-resist- 

 ing material ; and, further, its surface is always at a temperature 

 above the dew-point of the air. Vapour is therefore constantly 

 rising at night from the ground, and as this rising vapour has a 

 higher temperature and is saturated, it is better able to supply 

 moisture to cold surfaces than is the colder and dryer air above. 

 It will therefore be this rising vapour that will get condensed on 

 bodies near the earth. 



Dew may be deposited in a perfectly arid country, always pro- 

 vided that the radiating surfaces are not in heat communication 

 with the ground. There is always a possibility of a radiating 

 surface, exposed on a clear night in an arid climate, being cooled 

 by radiation to a temperature below the dew-point of the air. 

 In that case, the dew does not rise from the ground at the place, 

 but has risen long before, and been carried by the winds to where 

 it is deposited. 



In this climate, however, these are not the conditions under 

 which dew is deposited. If no vapour rose from the ground, 

 there is, no doubt, plenty in the air to give dew, and no doubt 

 some of the vapour in the air is deposited on bodies high above 

 the ground. But in our climate, in addition to the vapour in the 

 air, there is a constant supply of vapour rising from the ground 

 during the night, and, owing to the high temperature of the 

 ground, this rising vapour has a higher tension than that in the 

 air ; and, coming in contact with bodies near the ground, it is 

 naturally this rising vapour which we find deposited on them. 



But while we could have dew without this rising vapour, it 

 would not be so abundant, nor be deposited so early in the 

 evening. In that case it would be taken from the air above, and 

 not from the hot and saturated air from below. 



The Writer of the Article. 



Boulders in Coal. — At the April meeting of the Leeds Geo- 

 logical Association, Mr. C. Brownridge, F.G.S., read a short 

 paper, entitled "Notes on Four Boulders found in the Black Bed 

 Coal and overlying shales and ironstone at Wortley." Mr. 

 Brownridge, after alluding to the fact that the presence of 

 boulders in the coal measures is becoming an important ques- 

 tion, said that these interesting discoveries occur from time to 

 time, some having been found in the coalfields of Leicestershire, 

 Lancashire, and the Forest of Dean ; but none hitherto appear 

 to have been recorded from that immediate district. The posi- 

 tion where these boulders were found is situate in " No. I Black 

 Bed Pit.'' The whole of this neighbourhood is worked for the 

 Wortley fire-clay, and the depth of the black bed coal from the 

 surface is here 30 ft. The largest of the boulders is a coarse 

 gritstone, and nearly spherical in shape. Its dimensions are 

 2 ft. 6 in. by 2 ft., and it has a fairly smooth, polished face, with 

 slight strise. This example was found embedded in the " bind," 

 or clayey shales, just overlying the coal. The other three 

 boulders (or pebbles) are much smaller in size, varying from 

 II in. by 9 in. to 3|- in. by i\ in., and were all found embedded 

 in the black bed coal itself. One of the specimens is a fine- 

 grained grit, the other two being quartzites. The two latter are 

 rather more angular in general shape than the grit specimens, 

 but in all of them the angles are well rounded off and the faces 

 polished. The reason why these stones are thus found located in 

 such unusual positions can only at present be surmised, as 

 the subject is at present rather vague ; but the theory has been 

 adduced that they have been carried down by masses of floating 

 vegetation in a manner similar to that recorded by travellers on 

 the Amazon, where, in the swamps and shallows, such masses 

 are seen floating, carrying foreign matter along with them. 



Mechanical Reporting. — 'We understand from Industries 

 that Don Pedro Forres, the maker of the astronomical instru- 

 ments for the San Fernando Observatory, has completed a writing 

 machine, to which he has given the name of "prontuograph," 

 and which is intended to supersede shorthand writing. The 

 present operator can follow a speaker for two hours at the rate 

 of 130 words per minute, and it is stated that any one can do the 

 same with five or six months' practice. The advantage of this 

 machine over shorthand is that writing executed by it can be 

 read by any one after a quarter of an hour's instruction, and copy 

 can be sent direct to the compositors without having to be 

 transcribed. The machine resembles a small pianoforte, and 

 requires about the same amount of pressure. 



