Oct. 5, 1876] 



NATURE 



505 



the criticism on Prof. Blasius* recent book on storms may 

 be cited as an illustration, and a close adherence to its 

 text, viz., storms practically considered. 



LETTERS TO THE EDITOR 



[77ie Editor does not hold himself responsible for opinions expressed 

 by his correspondents. Neither can he undertake to return, 

 or to correspond with the writers of, rejected manuscripts. 

 No notice is taken of anonymous communications.^ 



Force 



In his valuable lecture on force at Glasgow, reported in 

 Nature, vol. xiv., p. 459, Prof. Tait did great service by 

 insisting on the duty of precision and consistency in the use of 

 this as of other scientific terms, and showed clearly how the word 

 "force " may be used precisely and consistendy. My reason for 

 troubling you with this communication is that I am unable to 

 identify this use of the word with Newton's, on the assumption 

 that the English equivalent for Newton's vis is " force." 



As the same difficulty has probably occurred to other readers 

 of Nature, T should be glad if Prof. Tait would kindly tell us 

 through your columns what are the equivalents in English for the 

 phrases (i) vis, (2) vis insita, (3) vis impressa, each of which is 

 used in Newton's " Principia." 



In the phrase vis insita — ii force is the English for vis — is not 

 a meaning of the word " force " implied which is wider than and 

 inclusive of the meaning of vis impressa ? P. T. Main 



An Intra-Mercurial Planet 



The discussion as to the existence of a planet within the orbit 

 of Mercury leads me to communicate an observation made many 

 years ago, which I believe nothing but the existence of an un- 

 known planet between us and the sun can explain. On Sunday, 

 January 29, i860, the sun rose in a fog in London, so that he could 

 be steadily looked at as if through a dark glass. Soon after 

 eight o'clock a perfectly round black object was seen by four 

 persons, including myself, clearly defined upon the lower half, 

 according to my recollection, of the sun's disc. It passed slowly 

 across his face and made its egress at about half-past nine a.m. 

 In apparent size it was equal to the representations I have seen 

 of Mercury in transit. F. A. R. Russell 



Pembroke Lodge, Richmond Park, September 30 



Brilliant Meteor 



The brilliant meteor of September 24 was well seen in the 

 neighbourhood of Ipswich, and as the observation of it was 

 difficult in the absence of stars, the following notes may be use- 

 ful. It was first seen at 6h. 31m. 15s. L.M.T., and the train 

 was visible as a luminous cloud until 6h. 47m. 3s. L.M.T. 

 The course had a length of about 25°, which was described in 

 three seconds, and made an angle of 80° with the horizon. By 

 means of the train which it left behind, it was possible to fix the 

 point of disappearance with considerable accuracy, namely : 

 altitude, 14° 6' ; azimuth reckoned from south towards east, 

 54° 16'. At this time Saturn was visible, having an altitude of 

 10° 56', azimuth 53° 15'. 



For purposes of description the course may be divided into three 

 portions, roughly equal. In the first portion the meteor had a 

 uniform brightness somewhat greater than a first magnitude star, 

 but during the second portion it rapidly increased to many times 

 the brightness of Venus, and almost suddenly diminished to its 

 former magnitude. In the third portion it again increased in 

 brilliancy, considerably exceeding its former maximum, and was 

 suddenly extinguished without bursting. This third portion only 

 was marked by the train estimated about 6° long, with a scarcely 

 perceptible breadth. During the sixteen minutes that the train 

 was visible it drifted about 12° northwards, losing gradually its 

 definite outline. Direction of wind, south-south-west. 



The diameter of the disc was certaiidy not greater than 2', 

 and the form was pear-shaped, though not very prolonged, 

 leaving the observer with the idea that the peculiarity of form 

 was merely due to the persistence of the impression on the 

 retina. It is very difficult to estimate its maximum brightness 

 accurately, as the heavens afford us no object with which to 

 compare it. I have recently shown that Venus has only g^th 

 part of the light of the full moon, and there is no other standard 



of light with which to bridge over this gap. If the moon had 

 only a diameter of 2', its intrinsic lustre would be 240 times greater 

 than it is, and the intensity would probably be such as would 

 cause the observer involuntardy to avert his eyes when seen sud- 

 denly, even in full twilight ; still, I do not thmk the meteor had 

 much less light than such an object would have. The glare was 

 of the colour, and closely resembled, a very vivid flash of light- 

 ning, for which it was mistaken by many persons. 



John I. Plummer 

 Orwell Park Observatory, September 27 



The Age of Palaeolithic Man 



In the extremely interesting communication on this subject 

 which Mr. Skertchley has made to Nature, vol. xiv. p. 448, 

 there are one or two points on which I should like to say a few 

 words. 



First, in approaching this subject and endeavouring to find 

 out the whole truth let us in starting have nothing but the truth, 

 A human bone, a fibula, was certainly found beneath glacial 

 clay in the Victoria Cave at Settle, but so far no implements 

 have turned up from that ancient horizon. This is a simple 

 inadvertence which does not in any way affect the strength of 

 Mr. Skertchley's position, but I am anxious to correct it and as 

 it were strangle it at the birth lest cuckoo-like it should shoulder 

 kindred but legitimate statements out into the cold. 



Mr. Skertchley's remarkable discovery consists in the finding 

 of palaeolithic implements beneath the great chalky boulder clay 

 of Mr. Searles V. Wood, jun., which is the so-called East 

 Anglian upper boulder clay, and this, as Mr Skertchley says, 

 and as I beheve Mr. Searles Wood holds, and with which I 

 certainly agree, is probably as old as the Lancashire lower 

 boulder clay or till. And this Lancashire till is undoubtedly of 

 the same age as the till of Scotland, as all authorities admit. 

 Moreover this till is generally admitted to be the product of the 

 great ice-sheet of Scotland and the North of England. We are 

 therefore landed at the conclusion that implements have been 

 found in beds which are probably of earlier age than the Scottish 

 ice-sheet, a conclusion in which I cannot but heartily concur. 

 Mr. Skertchley does not state this directly, but I presume this is 

 the legitimate inference to be drawn from his statements, and 

 one which he wotdd himself admit. 



There can be no doubt that this is very strong and corrobora- 

 tive evidence of the general views so ably urged by my friend, 

 Mr. James Geikie, that all palaeolithic implements and the fauna 

 associated with them are of inter-glacial age. It may seem 

 captious after having been led to the battle by so able a general, 

 and having driven the enemy so far [already, to grumble at his 

 stopping short in the pursuit, yet such is the object of my present 

 remarks. And I would wish to point out that there are heights, 

 or rather depths, which may yet be advantageously scaled to the 

 further discomfiture of the foe. 



Mr. J. Geikie has not ventured to carry the age of the bulk of 

 the palaeolithic beds further back than the time immediately suc- 

 ceeding the great Scottish ice-sheet. He appears to regard the 

 "great submergence" which followed this as the chief cause for 

 the removal from certain areas of the remains of men and animals 

 which peopled them in inter-glacial times. " The palaeolithic 

 gravels of the south-east of England . . . are contemporaneous 

 with those ancient valley-gravels of Scotland which overlie the 

 till and boulder-clay, and which are themselves partially re- 

 arranged and covered with marine deposits belonging to the time 

 of the great submergence."^ He certainly once " puts his hand 

 to the (ice-) plough." "No doubt, however, portions . . . espe- 

 cially in the districts south of the Thames, may date back to the 

 earlier warm periods of the glacial epoch, and thus be contem- 

 poraneous with the fresh- water beds in the Scottish till ; while 

 some may go back even to pre-glacial ages ; " but he imme - 

 diately "looks back" to the sea of the great submergence as the 

 great destroyer of palaeolithic records. " After the great ice- 

 sheet shrank back and the till and boulder clay had been de- 

 posited, a land-surface existed, rivers flowed down the valleys, and 

 plants and animals clothed and peopled the country. In Scotland 

 the fluviatile deposits belonging to that period have been subjected 

 to great denudation, but in one place at least they have yielded 

 animal remains, frogs and water-rats. But if the country had 

 never been submerged after the withdrawal of the ice from the 

 low grounds, there is good reason to believe that the presence of 

 the relics of palaeolithic man and remains of the animals with 

 * "The Great Ice Age," pp. 482-9. 



