JH plans to explore volcanic districts of Auvergne and Vivarais, then along Rhine to Bonne and Andermatte. Has cancelled plan to cross to Staffa [Scotland] and explore there.

Asks JH about Dr. Balfour's account in Asiatic Researches about lunar effects on humans in India.

Comments on Josef Fraunhofer's work on double stars, on quality and color of light from different sources, refraction of different kinds of light, and solution of a problem in conic sections. Is going to Geneva and Modena soon.

Describes observations made as he travelled and people visited on journey from London to Ancona.

Talks about Josef Fraunhofer's failing health and his making of flint glass.

Is sending on two items directed to JH through WT.

Returns house keys. Smooth passage across Channel. Met James Grahame's brother-in-law, Mr. Robison, who is going to Paris to be married. James South arranged carriage for JH's travels in France. Servant James Child is well and observed four criminals executed in London before departing. News of Beckwiths.

Journey from London and French countryside. Towns and Paris are vastly improved since 1801 visit. Charles Babbage and wife are in Paris. Received letters of introduction from Georges Cuvier, J. N. Nicollet, and J. P. Delessert for JH's expedition to Auvergne. Alexis Bouvard is recovering from fall. Georgianna Babbage has recovered and promised to write to MPH. [Letter continued 4 Sept. in Moulins:] Traveled more than 100 miles today. [Letter continued 5 Sept. in Clermont-Ferrand:] Massive flooding in Riom. Impressive view of volcanoes. [Letter continued 6 Sept.:] Will ascend Puy de Dôme today with two companions.

Two unwanted companions caused JH to waste two days in Clermont-Ferrand. Describes volcanoes and high winds on peaks. Actinometer experiments on Puy de Dôme. Hunting fossil bones of extinct animals.

Describes region around St. Flour, where servant James Child was seriously ill for two days. Low opinion of French physicians. Journey to Montpellier.

Tour delayed at least one week. Clouds have restricted JH's actinometer experiments on mountains. Describes active volcano and vineyards near Agde. Ancient ruins in Nismes [Nîmes].

Death of James Grahame's relative. Explored volcanoes in Vivarais Mountains. Describes Mont Blanc at sunset. Regrets bypassing St. Bernard. Expects to reach London by 13 Oct.

Plans leisure sightseeing in Paris. Visited Louvre. [P.S. 7 Oct.:] Called on Alexis Bouvard yesterday. Met W. H. Wollaston, returning from Tyrol. Wrote to Caroline Herschel from Montpellier. Will view A. M. Ampère's electrical experiments today.

Questions and comments regarding improvements in microscopy.

Comments on several experiments with spectra passing through glass films.

[Addition to Mary Baldwin's letter.] Health of JH's uncle. Travels through volcanic country of Vivarais. Asks CH to purchase for JH 10 copies of the first volume of William Herschel's works, recently translated into German. Sent to CH six copies of JH's 'Account of a Series of Observations Made with a 20-feet Reflecting Telescope' (1826) via Mr. Golterman, and six copies of JH's 'On the Parallax of the Fixed Stars' (1826) via Capt. Müller. James South received Copley Medal this year. Asks about comet near sun on 18 Nov.

Is making arrangements to meet with TH in London.

Tells CB of the mistake on the certificate for admission [to the R.S.L.?] for Mr. Moreau, which meant it had to be withdrawn. Doubts that reapplication would be successful. Other news from London.

Will send JH some barometric observations; describes the barometers with which the observations were made.

Discusses casting and construction of telescope speculae.