Feb. 27, 1SS5.] 



KNOWLEDGE ♦ 



181 



from twenty to forty miles an hour, which is probable enough. 

 But, he continues, "we have the testimony of captains ot some of 

 the transatlantic steamers that thev have oxperioni.L'd heavy scales 

 on the American coast which have ti-avelled with them all the way 

 to Europe with nuabatcd force." 



How is this possible? No steamer that over was built could 

 cross the Atlantic during a contiimous gale at an average speed of 

 thirty miles an hour, or even half that. 



The article concludes with the remark that " very much yet 

 remains to bo dono before we can call meteorology an exact 

 science." This :a indisputable. Sigma. 



[While I am absolutely in accord with "Sigma" in his conclud- 

 ing sentence, he seems to me to misapprehend the passages on 

 which he comments. For (1) The atmosphere undoubtedly has a 

 surface as sharply defined as that of the ocean itself ; and (2) I 

 cannot read the part he quotes with reference to the Atlantic gales 

 as indicating that a steamer has ever got into one particular pan 

 of a (more or less) cyclonic disturbance, and travelled with it 3,000 

 or 4,000 miles I The vessels may well have run into and out of 

 half-a-dozen vortical depressions travelling approximately in one 

 direction during her voyage. — Ed.]. 



PHOTOMICKOGRAPHY. 



1^1617] — In reviewing "Photo-Micrography" by A. C. Malley, 

 yoQ say that " many observei-s are sutliciontly export in sketching 

 with the camera lucida, the neutral tin plate, or SomnuMing's steel 

 disc; but nnfortunatcly their witness does not always agree 

 together, and their draughtsman will not unfrequently give very 

 different sketches of the same object." 



I met with a very striking illustration of the latter part of this 

 sentence last week, and I take the liberty of sending you the photo 

 and the notes I made upon it. I have done a little at photo- 

 micrography for some years, and feel sure you wonld do many of 

 your readers great service by showing its advantages. It was no 

 smaU trouble in the old collodion days, but it is simplicitj' itself 

 now, and the advantages are both manj' and great. 



The photograph represents one of the two mandibles of the 

 parasite of the beetle {Gamasiis Coleoptratorutn) . The length is 

 only the one sixty-sixth part of an inch, and it is only about the 

 one five-hundredth part of an inch in width. In the " Micrographic 

 Dictionary," a widely different representation is given of it from 

 that revealed by photography ; and it is not difficult to see that no 

 great harm could be done if the mandible were such as there 

 represented. Every one who has looked at the parasites which 

 cluster on the under surface of the beetle, must have wondered 

 liow they contrived to keep their hold upon a host so active, and 

 with a body so little likely to afford a safe footing; and then, if 

 able to keep their hold, how do they obtain their food from a 

 creature so well-armed as the beetle .'' An examination of the feet 

 of the Gamasii.^ Coleoptraturum shows that he will not be easily 

 shaken off; while the pair of mandibles with which it is armed 

 suggest the power it has of obtaining food. 



The part of the mandible here represented, when cut off and 

 mounted in Canada balsam, is only just visible to the naked eye, 

 but when enlarged, presents, with its finely pointed and gracefully 

 curved hooks, with their three teeth on each side, a sufficiently 

 formidable instrument of torture. George SoCTn.\LL. 



[This letter was accompanied by a photograph of the mandible 

 of the Gamasus coleoptraforum, and a fac-simile sketch of the 

 engraving of the same object in the "Micrographic Dictionary." 

 The very considerable difference between the natural and artificial 

 representations of the same object is, as Mr. Southall intimates, 

 very striking, and well illustrates the justice of our reviewer's 

 dictum on p. 134. — Ed.] 



THEEMO-CHEMISTEY. 



[1618] — It is a well-known fact that when two elements combine 

 heat is either rendered latent or latent heat is rendered apparent, 

 according as the combination of the atoms in the compound is (1) 

 less strong or (2) more strong than the combination of the atoms 

 in the elementary molecules. Has any good explanation been given 

 of this fact !' If not, does the following theory explain it? — That 

 since atoms of different elements have a greater attraction for one 

 another than atoms of the same element, the atoms in the molecule 

 of the compound will vibrate with smaller paths than they would 

 when existing in the molecules of the several elements forming the 

 compound. The energy representing the difference of these two 

 sets of vibrations would be given off in the form of heat. This 

 theory would explain the observed phenomena, since the stronger 

 the chemical compound the more heat is there developed on its 

 formation from its elements. Also, it is certain that, whatever the 



causo of tho heat whicli is given oil', this boat is pruduced by 

 changes inside tho moloculo. 



1 know not if the Theory of Vortex Atoms offers any solution of 

 these thermo. chemical dilliculties. Perhaps you could enlighten 

 mo on this point. Gilks Dm henkv. 



IX.SCIUHKD HOCKS. 



[1G19J — In going thi'ougli a field here a short time ago, 1 hap- 

 pened to cross one of those rocky knolls, common in this dia'.rict, 

 covered, all but a small patch on tho top, with a thin coating of 

 turf. On this bare spot I noticed what looked like jiart of a rough 

 circle close to the edge of tho grass. Turning back the turf, I 

 found a spiral figure about 15 in. in diamcfcr inscribed on the rock. 

 Further invesligation revealed a number of figures somewhat like 

 what 1 have drawn liclow, roughly cut and evidently much worn, 



in some parts almost gone. In the same field, at a distance of about 

 50 yards, I have since discovered another similarly-inscribed rock ; 

 and in a neighbouring parish, during one of last winter's storms, a 

 group of the same kind of markings was exposed on a flat rock 

 surface by the blowing over of a large tree. 



Can any of your arclueological readers inform me as to the origin 

 and use of these inscriptions ? Hugh McMeistek. 



THE DOCTKINB OF CHANCES. 



[1G20] — Whitaker's Almanac for this year contains an article on 

 "The Doctrine of Chances," whicli gives some clear information 

 on the subject in a condensed form. But tho illustration of the 

 subject numbered 9 at page 408 of the Almanac is curiously 

 erroneous. The so-called illustration is as follows : — 



A speaks truth three times out of four, B four times out of live, 

 C five times out of six. What is tlie chance of tho truth of an 

 event which A and B agree in asserting, but which C denies P The 

 event (according to Whitaker) happened, if A and B both speak 

 the truth and C lies ; it did not happen if A and B both lie and C 

 Bjieaks truth. Tho chance is therefore as follows (I am still quoting 

 Whitaker) i— 



This fraction is equal to -\f, therefore the odds are 12 to 5 in 

 favour of the event having taken place. 



Whitaker does not perceive that the system of multiplying the 

 fraction which represents one probability by the fraction represent- 

 ing another probability Is in no wise apiilicablo to the case which 

 he has created. vSuch a system would load to most absurd results. 

 For instance, if a million people testify to an event, common-sense 

 tells one that it is highly probable to be true. But the plan of 

 multiplying fractions representing each individual's credibility would 

 lead to a very minute product when a million persons are involved, 

 and would tend to show that a fact testified to b}' many witnesses, 

 instead of being highly probable, is just the reverse. 



The proper way of dealing with such a problem seems to be to 

 take for it the chance of both A and B lying. This is ][Xl—z'r,- 

 Therefore the chance of their speaking the truth is iji ^° that it 

 comes to weighing the words of a person who speaks tho truth 

 nineteen times out of twenty against that of another who speaks it 

 five times out of six. llodifying Whitaker's formula in this 

 manner, the odds come out 19 to 5 in favour of the event, instead 

 of 12 to 5. Chas. G. Su.\w. 



LETTERS RECEIVED AND SHORT ANSWERS. 



Jas. Bow.ma.'?. Shall have immediate attention. 1'he note was 

 part of the advertisement. The publishers had nothing whatever 

 to do with it. — M. F. W. sends a sketch of a pair of champagne 

 nippers, " registered Aug. 10, 1878," which he imagines to be 



