CONTENTS OF VOLUME L 



} 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 403 



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 425 



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 443 



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. Their 

 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 467 



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 Complexron. 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 Egss. 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 479 



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 499 



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 Cloud. 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. On 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 duality 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, <5cc. 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 Recom positions. 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. 



