May 4, 1883.] 



♦ KNOWLEDGE ♦ 



267 



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[Many correspondents of Ksowledge have, from the 

 beginning of its career, indicated a wish tliat we should 

 adopt something like the chatty style which (in response 

 to that frequently-expressed wish) wo have actually 

 employed in our Gossip columns. A little space has been 

 taken up in that way, which other correspondents grudge. 

 Like the alderman who regarded the margin round his 

 " proof before letters " as so much space wasted, these 

 0iatter-of-fact souls couat all which is not to be called 

 " knowledge simply worded and exactly described " as waste 

 or worse. Though few and ill-mannered, they mar the 

 tone of our converse with our readers. If a host who had 

 invited friends of scientitic tastes to meet hiui, should, in 

 his desire to entertain them, chat familiarly about other 

 than strictly scientific subjects — for which, nevertheless, he 

 left ample scope — and but a single guest, being rude of 

 bearing, should object, he would yield to that one guest, 

 though the rest should be more pleasantly disposed. For 

 who cares to be familiar with his friends when even but one 

 of the gloomily grumbling sort is present ? For this reason 

 we must ask the many kindly friends who have expressed 

 pleasant approval of our Gossip to forgive us if we no longer 

 accede to their wishes in regard to it. The dull souls 

 who count every Gossip paragraph as so much stolen from 

 them (but they never considci- how generously they are 

 dealt with in original matter, of which they get thrice as 

 much here as in a magazine of thrice our cost), shall have 

 their way. There will iu future be no Gossip. We beg 

 that they will tiol be at the pains to express their approval ; 

 and we should be ratlior glad if the much more numerous 

 readers of the kindlier sort would let the matter pass 

 without comment by letter, as our correspondence is falling 

 dismally into arrears.] 



COMETS' TAILS. 



[802] — Mr. Eanyard is doubtless right in Lis observation tbat the 

 discharge of gas from each stone of a meteoric swarm is greatest 

 from the side tamed towards the snn ; but I do not think that the 

 Icinal direction of motion in the cage of each gaseous molecule, at 

 the instant at which it becomes an independent projectile launched 

 into space, can be much affected by this. The gas is not in a 

 gaseous form while in the stone, but it takes this form at tho instant 

 of departure, and retains it until its volume is so far expanded that 

 the molecules part company. While the gaseous form is retained 

 the molecules are necessarily moving in equal numbers in all direc- 

 tidns, by reason of their mutual encounters, whatever the initial 

 direction of the impulse which shook them from their occluded 



state may have been; and, as any portion of tho gas must expand 

 to several million times the volume it occupied while in tho stone 

 before its molecules become independent of each other, the greater 

 number of tho molecules must at this time be quite independent of 

 the stone. 



The quantity of matter in a, comet's tail must be very small, 

 whatever his origin ; but we can scarcely know what density is 

 necessary to make a vast e.xtont of gaseous matter, or of separated 

 molecules, visible. Every molecule probably reflects a portion of 

 tho light falling on it, and though the effect upon our eyes may be 

 insensible, and tho result, therefore, uitcr blackness when tho 

 volume is a few cubic feet, it may be otherwise when we look 

 through millions of cubic miles. I think, however, with Mr. 

 lianyard, that tho presence of particles of tho nature of snioko or 

 dust may be inferred, and it seems probable that occluded gases 

 when escaping would carry with them in molecular combination, 

 and would afterwards precipitate minute portions of the materials 

 from wliich they escape. Albert J. Mott. 



TIDE AND WEATHER. 



[^803 J — From observations made in London, and more especially 

 in a coasting district, I can confltlently assert that the state of the 

 weather is always affected by high and low water, and that a 

 change to a greater or less degree (sometimes almost inappreciable) 

 takes place at, or a short period before, tho turn of the tide. 



RoiiERT Connell, M.D. 



THE GREAT PYRAMID. 



; 804] — Pray allow mo to thank you for your most interesting 

 book on the Great Pyramid, and excuse my troubling you with a 

 .speculative guess respecting the so-called ventilators. 



1. They can hardly have been solely for purposes of ventilation ; 

 for, if so, an immense amount of careful work might have been 

 spared by making them perpendicular, as then it would only have 

 been necessary to cut off the corners of the blocks of stone, 

 and to build them up so that the cut comers should coincide in tho 

 successive layers, thus — 



Horizontal Section. — Ventilator at A. 



Instead of this, the tubes go in a slanting direction through the. 

 blocks, and must have been both difhcult to cut and especially bad 

 to join in the consecutive blocks. 



B^ 

 *^-'''' 



Vertical Section. — Ventilator, A B. 



2. From this I argue that they had some special — probably 

 astronomical — use, and my guess is that the northern tube was to 

 keep the orientation of the Pyramid correct in the upper layers ; 

 and that tho southern tube was intended to observe the elevation of 

 the sun when on the meridian at the winter solstice. 



3. I'erhaps you may be able to say if the northern ventilating 

 tube points to the north in such a way as to preserve the orienta- 

 tion. 



4. The southern ventilating tube was, I imagine, intended to 

 solve this problem — Is the inclination of the earth's axis to the 

 plane of the eclipse a constant quantity, or (as the problem would 

 present itself to the ancient astronomers) are the maximum and 

 minimum angular elevations of tho sun at noon above tho horizon 

 (on longest and shortest days respectively) constant quantities, or 

 are they slowly changing ? 



To resolve this problem, at first they may have accurately 

 measured the shadow cast by the sun at noon on shortest day in 

 succestive years. This method, however, would soon be seen not 

 to be sufficiently accurate to detect any change. A tube, down 

 which the sun would shine only on the one day of the year, and 

 which could be maintained in its exact position for three or four 

 thousand years, would bo a sufficient test ; for (1,) if the axis of the 

 earth became more nearly perpendicular, the sim would no longer 



