CONTENTS OF VOLUME I. 



Comets. — Biela's Comet. — Its Period and Orbit — Lexell's Comet— Causes of its Appearance and 

 Disappearance.— Whiston's Comet.— His Theory. — Did this Comet produce the Deluge ?— Orbit 

 of this Comet. 



RADIATION OF HEAT page 435 



Radiation a Property of Heat. — Prismatic Spectrum. — Invisible Rays. — Two Hypotheses. — Invis- 

 ible Rays alike in their Properties to luminous Rays. — Discoveries of Leslie. — Differential Ther- 

 mometer. — Radiation, Reflection, and Absorption. — Effect of Screens. — Supposed Rays of Cold. — 

 Common Phenomenon explained. — Theory of Dew. 



METEORIC STONES AND SHOOTING STARS 457 



Inductive Method.— Appearances accompanying Meteorites. — Theories to explain them.— Exam- 

 ination of these Theories. — Shooting Stars. — November and August Meteors. — Orbits and Dis- 

 tances.— Heights. — Chladni's Hypothesis. 



THE EARTH , 475 



A difficult Subject of Investigation. — Form of the Earth.— How proved globular.— Its Magnitude. — 

 Its annual Motion.— Elliptic Form of its Orbit.— Proofs of its annual Motion from the Theory of 

 Gravitation. — From the Motion of Light. — The Earth's diurnal Motion. — Inequalities of Day and 

 Night. — Weight of the Earth. — Maskelyne's Experiment. — Cavendish's Experiment. — Tbeir 

 Accordance. — Density of the Earth. — The Seasons. — Calorific Effect of the Sun's Rays. — Why 

 the longest is not the hottest Day. — Why the shortest Day is not the coldest. — The hottest Season 

 takes place when the Sun is farthest from the Earth. — Proofs of the diurnal Rotation. — Spheroidal 

 Form of the Earth proved by Theory and by Observation. 



LUNAR INFLUENCES 499 



The red Moon. — Supposed Effect of the Moon on the Movement of Sap in Plants. — Prejudice 

 respecting the time for felling Timber. — Extent of this Prejudice. — Its Prevalence among Trans- 

 atlantic People. — Prejudices respecting Effects on Grain. — On Wine. — On the Complexion. — On 

 Putrefaction. — On Wounds. — On the size of Oysters and Shellfish. — On the Marrow of Animals. — 

 On the Weight of the human Body. — On the Time of Births. — On the Hatching of Eggs. — On 

 Human Maladies. — On Insanity. — On Fevers. — On Epidemics. — Case of Vallisnieri. — Case of 

 Bacon. — On Cutaneous Diseases, Convulsions, Paralysis, Epilepsy, &c. — Observations of Dr. 

 Olbers. 



PHYSICAL CONSTITUTION OF COMETS 511 



Orbitual Motion of Comets. — Their Number. — Their Light. — Explanation of this. — Theory of Her- 

 schel.— Constitution of Comets.— Nebulosity.— Nucleus.— Tail.— Comets of 1811—1680—1769— 

 1744—1843—1844. 



THUNDER-STORMS 529 



The Deficiency of our present Knowledge. — Of common Thunder- Clouds. — Character and electric 

 Charge of Clouds. — Discharge between vicinal Clouds. — Conditions for such Discharge. — Dis- 

 charge between the Clouds and the Earth. — Mutual Attraction or Repulsion of electrized Clouds. 

 — Characters of the upper and of the lower Surface of Clouds. — Negative Testimony respecting 

 Thunder from an isolated Cloud. — Cases of Lightning from an isolated Cloifd. — A fresh Case 

 related by M. Duperrey. — Obvious Inferences from the above Cases. — Of volcanic Thunder- 

 Clouds. — Lightning from the Ashes, Smoke, and Vapor of Volcanoes. — Theoretical Ideas of its 

 Origin. — Of the Height of stormy Clouds. — Mode of Observation. — Ascending Flashes of Light- 

 ning. — Minor Limits of the Height of Storm- Clouds. — Inefficiency of many recorded Observations. 

 — Table of Observations as collected by Arago. — Flash of Lightning from a Cloud upward. — Of 

 Lightning. — Varieties of Lightning. — Zigzag Lightning. — Forked Lightning. — Deficiency in our 

 Vocabulary of Terms. — Sheet Lightning. — Table of Instances of Ball-Lightning. — Mr. Harris's 

 Explanation of Ball-Lightning.— Ore the Speed of Lightning.— -Theory of Vision illustrated by 

 a rotating Disk. — Wheatstone's Experiments. — Observations of the Velocity of Lightning. — 

 Silent Lightning. — Heat Lightning. — Thunder-Bursts. — Of luminous Clouds. — Clouds them- 

 selves faintly luminous. — Possession of the Quality in various Degrees. — Clouds visibly luminous. 

 — Various Observations of luminous Clouds. — Sabine's Observations. — Of Thunder. — Rolling 

 of Thunder. — Duration and Intensity of rolling Thunder. — Violent Thunder from Ball-Lightning. — 

 Interval between Lightning and Thunder. — A Case in which they were almost simultaneous. — 

 Thunder without Lightning. — Noise attendant on Earthquakes. — Of the Attempts to explain the 

 Phenomena of Thunder and Lightning. — Identity of Lightning and Electricity. — Whether pon- 

 derable Matter, or a Propagation of Undulations. — Difficulties of the undulatory Hypothesis. — 

 Ball-Lightning and the Inferences to which it leads. — Bituminous Matter accompanying a Case 

 of Lightning Discharge. — Explanations of silent Lightnings. — Observations of silent Lightnings. — 

 Difficulties in the Explanation of silent Lightnings. — Arago's Suggestion for Observations. — 

 Lightning hidden by dense Clouds.— Place of the Sound of Thunder.— Greatest Distance at 

 which Thunder is heard. — Case of Distance beyond which it was inaudible. — Distance at which 

 other Sounds have been heard.— Effects of Heat, Cold, Wind, &c — On the Transmission of 

 Sound.— Thunder heard when no Cloud was visible.— Hypothesis of the Cause of Thunder from 

 the Creation of a Vacuum. — Contractions and Dilatations of the Air assigned as the Cause. — 

 Pouillet's Theory of Decompositions and Recompositions. — Influence of Echo in causing the 

 Roll. — Duration of an Echo. — Duration of the Roll of Thunder at Sea. — Dr. Robison's Explana- 

 tion of the Roll.— Application of the Theory to Zigzag Lightning.— Inefficiency of the Theory. — 

 Means of obtaining a minor Limit of the Length of a Flash. 



