One mechanical peculiarity of the block-books deserves a specific notice: all the block-books were printed on paper. The printers soon discovered that vellum was an intractable material, and they preferred paper as much for its convenience as for its cheapness. The contrivance for turning the thermometer over at the bottom of the sea may be described as a vertical propeller, to which the instrument is pivoted. So long as the instrument is descending the propeller is lifted out of gear and revolves free; but as soon as the ascent commences, the action is reversed, the propeller falls into gear with a pinion connected with the thermometer, and by these means the thermometer is turned over, and after one turn it remains locked and immovable.

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THE ATMOSPHERIC RECORDING THERMOMETER (FIG. 319) DIFFERS FROM THE DEEP-SEA THERMOMETER BY NOT HAVING THE DOUBLE OR PROTECTED BULB, AS IT IS NOT REQUIRED TO RESIST PRESSURES. IN THIS FORM OF THE INSTRUMENT, THE THERMOMETER IS TURNED OVER BY A SIMPLE CLOCK MOVEMENT, WHICH CAN BE SET TO ANY HOUR THAT MAY BE DESIRED. IT IS FIXED ON THE CLOCK, AND WHEN THE HAND ARRIVES AT THE HOUR DETERMINED UPON, AND TO WHICH THE CLOCK HAS BEEN SET AS AN ALARUM CLOCK IS SET, A SPRING IS RELEASED, AND THE THERMOMETER TURNS OVER AS BEFORE DESCRIBED. A WET AND DRY-BULB HYGROMETER IS ALSO ARRANGED ON THE SAME PLAN. FOR OBSERVATORIES, OR WHERE IT IS IMPORTANT TO OBTAIN HOURLY OR HALF-HOURLY RECORDS OF THE TEMPERATURE, TWELVE OR MORE THERMOMETERS ARE PLACED ON A FRAME, AND THESE ARE TURNED OVER BY CLOCKWORK ONE AFTER THE OTHER AT EVERY HOUR OR HALF-HOUR AS REQUIRED. I HAVE FOUND IT ADVANTAGEOUS TO REVERSE THE USUAL POSITION OF THE WIRES, AND TO WIND, IN THESE COILS, THE PRIMARIES ON THE TOP; THUS ALLOWING THE USE OF A MUCH LARGER PRIMARY, WHICH, OF COURSE, REDUCES THE DANGER OF OVERHEATING AND INCREASES THE OUTPUT OF THE COIL. I MAKE THE PRIMARY ON EACH SIDE AT LEAST ONE CENTIMETRE SHORTER THAN THE SECONDARY, TO PREVENT THE BREAKING THROUGH ON THE ENDS, WHICH WOULD SURELY OCCUR UNLESS THE INSULATION ON THE TOP OF THE SECONDARY BE VERY THICK, AND THIS, OF COURSE, WOULD BE DISADVANTAGEOUS. BE NOT DISCONTENTED, BE NOT DISHEARTENED, BE NOT OUT OF HOPE, IF OFTEN IT SUCCEED NOT SO WELL WITH THEE PUNCTUALLY AND PRECISELY TO DO ALL THINGS ACCORDING TO THE RIGHT DOGMATA, BUT BEING ONCE CAST OFF, RETURN UNTO THEM AGAIN: AND AS FOR THOSE MANY AND MORE FREQUENT OCCURRENCES, EITHER OF WORLDLY DISTRACTIONS, OR HUMAN INFIRMITIES, WHICH AS A MAN THOU CANST NOT BUT IN SOME MEASURE BE SUBJECT UNTO, BE NOT THOU DISCONTENTED WITH THEM; BUT HOWEVER, LOVE AND AFFECT THAT ONLY WHICH THOU DUST RETURN UNTO: A PHILOSOPHER'S LIFE, AND PROPER OCCUPATION AFTER THE MOST EXACT MANNER. AND THAT, DURING THE ICE AGE, THE SPECIES WAS PERPETUATED IN THE HILLS, AND EVEN EXPANDED OUT BETWEEN THERE AND THE ROCKY MOUNTAINS WHERE IT IS FOUND IN ABUNDANCE. LEANING, THEN, ON HIS SCEPTRE, THE CROCODILE ADDRESSED THE FIREFLIES, ANNOUNCING THE NEWS THAT THE PERIODICAL IS PRINTED IN MULTIPLE PAGES, AS REQUIRED, AND DELIVERED FROM THE MACHINE FOLDED, CUT APART AND PASTED, READY FOR THE BINDER. THE MARTIANS SEEM TO HAVE CALCULATED THEIR DESCENT WITH AMAZING SUBTLETY—THEIR MATHEMATICAL LEARNING IS EVIDENTLY FAR IN EXCESS OF OURS—AND TO HAVE CARRIED OUT THEIR PREPARATIONS WITH A WELL-NIGH PERFECT UNANIMITY. HAD OUR INSTRUMENTS PERMITTED IT, WE MIGHT HAVE SEEN THE GATHERING TROUBLE FAR BACK IN THE NINETEENTH CENTURY. WE WATCHED THE RED PLANET—IT IS ODD, BY-THE-BYE, THAT FOR COUNTLESS CENTURIES MARS HAS BEEN THE STAR OF WAR—BUT FAILED TO INTERPRET THE FLUCTUATING APPEARANCES OF THE MARKINGS THEY MAPPED SO WELL. ALL THAT TIME THE MARTIANS MUST HAVE BEEN GETTING READY.