98 



POPULAR SCIENCE l^EWS. 



[July, 1S89. 



with a force amounting to many tons, but, as 

 it is exerted in all directions, both from the 

 inside and ontside of our bodies, we are not 

 conscious of it. 



The pressure of the air is not constant, as 

 would be the case with a column of water, 

 but varies considerably from time to time, 

 owing, principally, to air currents of varying 

 temperatures and consequent density, and 

 similar meteorological causes. The average 

 pressure may be conveniently stated as fifteen 

 pounds to the square inch, or a pressure suf- 

 ficient to sustain a column of mercury about 

 thirty inches high. 



A barometer is simply an instrument for 

 measvning the varying pressure of the air, 

 and in its simplest form consists of a glass 

 tube, closed at one end, and filled with mer- 

 cury, the lower open end of which dips into a 

 vessel of the same metal, and provided with a 

 scale to measure the height of the mercurial 

 column. If the tube is more than thirty 

 inches long, the mercury will fall away from 

 the top, leaving a vacant space, which will 

 vary in size from time to time, as the varying 

 atmospheric pressure will balance the weight 

 of a longer or shorter column of mercury. 

 The einpty space at the top of the tube is 

 called the Toricellian vacuum, and is a very 

 perfect one, containing only a little vapor of 

 mercury. 



The popidar notion of a barometer is that 

 of a weather prophet, and it is assumed that 

 its indications are uniform and certain in 

 every case. Nothing could be further from 

 the fact. It is true that the state of the 

 weather is, to a large degree, dependent upon 

 or varies with the atmospheric pressure, but 

 the barometer simply indicates these varia- 

 tions of pressure, and their interpretation in 

 connection with meteorological changes must 

 be a matter of special study, and largely de- 

 pendent upon local conditions. As a general 

 rule, it may be said that an increasing pres- 

 sure indicates the approach or continuance of 

 fine weather, while a decreasing pressure pre- 

 cedes a storm or atmospheric disturbance, but 

 this rule is by no means infallible, and not 

 infrequently the reverse might be the case. 

 It requires a skillful observer to properly in- 

 terpret the barometric readings, and this is an 

 unavoidable source of error in the government 

 predictions, which are made up at a central 

 point from observations transmitted by tele- 

 graph, and by a single oflicial, who has no 

 means of making allowance for movements 

 due to local conditions alone. 



The actual manufacture of a barometer is a 

 matter of considerable care and skill, a really 

 good instrument costing from thirty dollars 

 upward. The cheaper ones are equally good 

 for noting the comparative changes of pres- 

 sure, which is all that is usually required, but 

 they are utterly worthless for giving the actual 

 pressure. Great care must be taken in filling 



the tube, and the mercury is added in small 

 quantities and kept at the boiling point (662*^ 

 F.) during the process, to drive out the least 

 traces of air. The instnmients used for sci- 

 entific-purposes are provided with an accurate 

 scale of inches and fractions, showing the 

 absolute height of the top of the mercury col- 

 umn above the surface of the mercury in the 

 reservoir at the base, and a vernier, by which 

 variations as small as the thousandth part of 

 an inch may be read of!". Such an instnunent 

 would show the variations of pressure be- 

 tween two different floors of a building. 



It is by no means a simple matter to take 

 an accurate barometric observation. Where 

 absolute accuracy is required, the apparent 

 height must be corrected, for the latitude of 

 the place of observation, for the temperature 

 of the air, for the expansion of the material 

 on which the scale is engraved, for the capil- 

 larity of the tube, and for the height above 

 sea level. Usually the latter correction only 

 is necessary, but, in certain investigations, all 

 these and others must be allowed for. 



Barometers are used to measure the pres- 

 sure of the air, or other gases and vapors, in 

 various physical and chemical investigations, 

 and they play > a very important part in the 

 determination of the specific gravity and con- 

 sequent molecular weight of different volatile 

 chemical substances. One of their most im- 

 portant practical uses is in the determination- 

 of the height of mountain ranges, hills, etc. 

 It is evident that the higher we ascend into 

 the air, the less the weight above us, and, 

 consequently, the less the pressure upon the 

 mercurial column. Two barometers should 

 be used, and observations taken simultaneously 

 at the base and summit of the elevation, un- 

 less the time between the two observations is 

 short. For ordinary purposes, it is accurate 

 enough to reckon an elevation of 93 feet for 

 every tenth of an inch depression of the mer- 

 cury, but a more accurate formula for heights 

 not exceeding 3,000 feet is : 



X=52494.3 X 



( T+Ti 

 ■+ 



H+Hi ( 900 



X equals the elevation in feet, H the height 

 of the barometer at the lower station, and H i 

 the height at the upper station. T and Ti are 

 the temperatures of the respective stations in 

 numbers of Fahrenheit degrees above 32*^. 



Aneroid barometers depend upon the same 

 principle as the mercurial ones, but the at- 

 mospheric pressure is exerted upon a thin 

 corrugated metal box, oiie side of which is 

 connected by a system of gearing and levers 

 with a pointer moving over a scale. The 

 varying pressure causes the side of the box to 

 rise and fall, and the movement is transmitted 

 to the pointer. For ordinary purposes, 

 aneroid barometers are sufficiently accurate 

 and much more convenient than mercurial 

 ones, but, where extreme accuracy is re- 

 quired, they cannot be used. 



A cheap form of barometer, or weather indi- 

 cator, is extensively sold, consisting of a 

 sealed glass tube containmg a liquid in which 

 some solid matter is partially dissolved. 

 They are not of the slightest value, and are 

 not affected in any way by a change in atmos- 

 pheric pressure. The little "weather houses," 

 where figures move in and out of different 

 doors, depend upon the twisting or untwist- 

 ing of the string of catgut from which the 

 images are suspended. They are simply a 

 rude form of hygrometer, which only indicates 

 the amount of moisture present in the air, and 

 are very imreliable weather prophets. 



[Original in The Popular A'cience News J 

 EVOLUTION. 



BY PROFESSOR JAMES H. STOLLER. 



CONCLUDING PAPER. 



THE CAUSES OF ORGANIC EVOLUTION. 



We have seen that the study of the organic king- 

 dom, from the point of view of any of the several 

 sciences which relate to the subject, brings to light 

 a vast array of facts which are intelligible only upon 

 a theory of organic evolution. Now, while it is 

 entirely competent to urge the truth of such a theory 

 solely upon the basis of the facts explained by it, 

 yet it becomes desirable to show clearly the nature 

 of the assumed process. In other words, when it 

 appears to us that a great number of definitely 

 observed facts render it impossible not to conclude 

 that there has been a process of evolution in organic 

 nature, we not only have a right to advocate this 

 view, but we also fairly incur the task of explaining 

 how the process was carried on, — in a word, what 

 the causes of the evolution were. 



It was to this task that the labor of Darwin was 

 directed. It was not, as generally supposed, that 

 Darwin proposed a theory of evolution of the or- 

 ganic kingdom, but that he set forth an hypothesis 

 to explain what he believed to be the great fact of 

 evolution. It is important to understand that evo- 

 lution, as a scientific doctrine or theory, is an 

 entirely different matter from Darwin's hypothesis 

 of natural selection. The former rests upon grounds 

 altogether distinct from the latter. Darwinism 

 might be entirely false, and yet evolution entirely 

 true. The evidences of evolution are the facts of 

 plant and animal structure, geological succession, 

 etc., noticed in the last paper; the evidences of a 

 principle of natural selection, as set forth by Dar- 

 win, are facts in regard to the habits, relations, etc., 

 of plants and animals. It is true that if Darwin's 

 hypothesis is proved true, organic evolution is no 

 longer a theory, but is established as a great law of 

 nature, and it is because Darwin succeeded so well 

 in bringing forward evidence of the truth of his 

 hypothesis that he is regarded as the founder of the 

 great doctrine of modern science. 



Natural Selection. — What, then, is natural selec- 

 tion, and in what way has it been determinative of 

 the evolution of organic forms.' We shall be able 

 to pursue our inquiry by considering certain aspects 

 of plant and animal life that come within common 

 observation. 



I. Any plant produces many more seeds than 

 can possibly give rise to new plants. Indeed, since 

 every available inch of soil is always occupied by 

 vegetation, it is clear that, so long as species main- 

 tain the same relative abundance, only one seed 

 from each plant can reproduce its kind. In the 

 case, then, of a plant flinging a hundred seeds to tlie 

 wind, what will determine which one or two seeds 



