On the Connexion of the Physical Sciences — A Reader’s Guide

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Authors: Somerville, Mary, 1780-1872 Digital Source: Project Gutenberg Gutenberg Release Year: 2016 First Publication Year: Not confirmed

Key Word: Science; Physical sciences Target Reader: Readers of public-domain and historical texts

Book Edition: Project Gutenberg digital edition Book Language: en

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Words 219,988

Reading time 957 min

Text sections 48

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Source: Project Gutenberg record #52869

Mary Somerville's 1858 ninth edition systematically demonstrates the interdependence of physical laws, using precise examples like pendulum compensation, crystal expansion, and heat propagation to show how disparate phenomena are governed by unified principles.

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e position with regard to the sun and moon, and occasions inequalities in the moon’s motion, which are more considerable than those arising from their direct action; for the same reason the moon, by disturbing the earth, indirectly disturbs her own motion. Neither the excentricity of the lunar orbit, nor its mean inclination to the plane of the ecliptic, have experienced any changes from secular inequalities; for, although the mean action of the sun on the moon depends upon the inclination of the lunar orbit to the ecliptic, and the position of the ecliptic is subject to a secular inequality, yet analysis shows that it does not occasion a secular variation in the inclination of the lunar orbit, because the action of the sun constantly brings the moon’s orbit to the same inclination to the ecliptic. The mean motion, the nodes, and the perigee, however, are subject to very remarkable variations.

From the eclipse observed at Babylon, on the 19th of March, seven hundred and twenty-one years before the Christian era, the place of the moon is known from that of the sun at the instant of opposition (N. 83), whence her mean longitude may be found. But the comparison of this mean longitude with another mean longitude, computed back for the instant of the eclipse from modern observations, shows that the moon performs her revolution round the earth more rapidly and in a shorter time now than she did formerly, and that the acceleration in her mean motion has been increasing from age to age as the square of the time (N. 105). All ancient and intermediate eclipses confirm this result. As the mean motions of the planets have no secular inequalities, this seemed to be an unaccountable anomaly. It was at one time attributed to the resistance of an ethereal medium pervading space, and at another to the successive transmission of the gravitating force. But, as La Place proved that neither of these causes, even if they exist, have any influence on the motions of the lunar perigee (N. 102) or nodes, they could not affect the mean motion; a variation in the mean motion from such causes being inseparably connected with variations in the motions of the perigee and nodes. That great mathematician, in studying the theory of Jupiter’s satellites, perceived that the secular variation in the elements of Jupiter’s orbit, from the action of the planets, occasions corresponding changes in the motions of the satellites, which led him to suspect that the acceleration in the mean motion of the moon might be connected with the secular variation in the excentricity of the terrestrial orbit. Analysis has shown that he assigned the true cause of the acceleration.

It is proved that the greater the excentricity of the terrestrial orbit, the greater is the disturbing action of the sun on the moon. Now, as the excentricity has been decreasing for ages, the effect of the sun in disturbing the moon has been diminishing during that time. Consequently the attraction of the earth has had a more and more powerful effect on the moon, and has been continually diminishing the size of the lunar orbit. So that the moon’s velocity has been gradually augmenting for many centuries to balance the increase of the earth’s attraction. This secular increase in the moon’s velocity is called the Acceleration, a name peculiarly appropriate at present, and which will continue to be so for a vast number of ages; because, as long as the earth’s excentricity diminishes, the moon’s mean motion will be accelerated; but when the excentricity has passed its minimum, and begins to increase, the mean motion will be retarded from age to age. The secular acceleration is now about 11ʺ·9, but its effect on the moon’s place increases as the square of the time (N. 106). It is remarkable that the action of the planets, thus reflected by the sun to the moon, is much more sensible than their direct action either on the earth or moon. The secular diminution in the excentricity, which has not altered the equation of the centre of the sun by eight minutes since the earliest recorded eclipses, has produced a variation of about 1° 48ʹ in the moon’s longitude, and of 7° 12ʹ in her mean anomaly (N. 107).

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Mary Somerville opens her ninth edition with a Baconian epigraph asserting that no natural phenomenon can be studied in isolation. The book then proceeds through forty-eight sections, each building a web of connections among astronomy, mechanics, optics, and heat. Somerville’s method is to move from a concrete problem—such as the effect of temperature on a pendulum’s length—to the broader principle it illustrates, then to its application in other fields. This structural pattern, repeated throughout, makes the work a sustained demonstration of how the physical sciences cohere.

From Pendulums to Planetary Motion

Somerville frequently uses a single device to link terrestrial and celestial mechanics. In the excerpt on clock pendulums, she describes Graham’s mercury compensation and Harrison’s gridiron pendulum, then immediately notes that the same principles of thermal expansion govern chronometers used in navigation and the compensation rods essential for surveying. The paragraph concludes by asserting that these laws “have an immediate influence upon our estimation of time; of the motions of bodies in the heavens, and of their fall upon the earth.” This telescoping from a workshop contrivance to the figure of the globe and the system of weights and measures exemplifies her connective method.

Crystalline Contradictions

A striking instance of Somerville’s attention to structure appears in her discussion of crystal expansion. She reports Mitscherlich’s finding that Iceland spar dilates along its optical axis but contracts at right angles to it, “which brings the crystal nearer to the form of the cube and diminishes its double refractive power.” She then describes how heat causes the two optical axes of sulphate of lime to approach, coincide, and then reopen at right angles. These observations are not isolated curiosities; she uses them to introduce Senarmont’s conclusion that isothermal surfaces in such crystals are concentric ellipsoids. The internal structure of crystallized matter, she remarks, “must be very peculiar thus to modify the expansive power of heat.”

Heat’s Ascent and the Cold of Alpine Lakes

Somerville shifts from solids to fluids with a concise explanation of heat propagation. Heat applied to a fluid’s surface penetrates slowly because warmer, lighter strata remain on top; this, she notes, is “the reason why the water at the bottom of lakes fed from Alpine chains is so cold.” When heat is applied below, particles rise as they become lighter, diffusing heat through the mass. She adds a comparative detail: mercury conducts heat twice as fast as water, “and therefore it appears to be very cold.” The passage moves from a physical principle to a geographical observation to a sensory impression, all within a few sentences.

The Architecture of the Solar System

The table of contents reveals the book’s overarching structure. Early sections treat attraction, elliptical motion, and perturbations, establishing the mathematical framework. Later sections apply this framework to Jupiter’s satellites, the figure of the planets, and the invariable plane of the solar system. Somerville repeatedly emphasizes that the stability of the system depends on “the Primitive Momentum of the Bodies.” The section headings themselves form a map of connections: “Universal Gravitation the Cause of Perturbations,” “Mean Motion and Major Axis Invariable,” “Effects of a Resisting Medium.” Each heading promises a link between a specific force and a global consequence.

Readers approaching this work will find that Somerville’s prose rewards attention to her transitions. She rarely states a principle without first grounding it in a tangible example—a pendulum, a crystal, a lake. By tracing how each example reappears in different contexts, one can follow the connective threads she weaves across the sciences. The ninth edition, revised after decades of scientific change, also offers a snapshot of mid-nineteenth-century physics in the midst of its own reconfiguration.

I kept thinking about how Mary Somerville shows everything is quietly connected—pendulums, crystals, heat—all obeying the same gentle laws. It stayed with me like a half-remembered tune. That same sense of hidden order came back while reading The platinum metals — A Closer Reading. The way those dense metals hold their own steady patterns felt like a small, patient echo of her larger vision.