| Description | He has many things to write about. He has mentioned [John Baboneau Nickterlien] Hennessey before, hoping that he would be made welcome if he visited Collingwood. John hopes that his father's opinion of Hennessey will be a favourable one. Hennessey will be a stranger in England, his knowledge of works of art and skill being limited to reading and conversations with those in India. His knowledge is considerably higher than could be expected. Hennessey aims to overcome this with a tour of manufactories, but is shy of asking favours, such as letters of introduction. [James Thomas] Walker has given him letters to [George Biddell] Airy and [Henry] James which may be of use, but John asks his father to give Hennessey personal introductions. He gives details of Hennessey's travelling times and arrival, and his promise to visit Collingwood. Hennessey is in deep mourning for his favourite sister, the third he has lost of four, and he is himself ill. John hopes his father has received a paper from him explaining his difficulty with the Nautical Almanac rules for finding a star's true place for any day. This is of much importance to their observed azimuths, where they may use stars not included the Almanac, where positions have to be calculated. His history of geodesy in India is in abeyance and he describes the difficulties of working with the manuscript records, believing that it will take years to accumulate enough knowledge to complete the work. He notes the sweeping reforms in the series: [George] Everest made great strides but his successors were fearful of meddling and as a result their methods have become antiquated. Walker is a radical in this and John gives an assessment of his abilities and character. He is a good mathematician, but John occasionally disagrees with him, giving an example where he involuntarily exclaimed in the presence of subordinates, angering Walker. He has been careful since and wonders at the wisdom of criticising Walker on paper. He returns to the reforms, those that are mathematical coming under John's notice. There was no system of corrections before Everest, and his were erroneous, if practical. John wishes to improve this but on examining the question sees many difficulties. Airy's little book is invaluable, as is the 'Account of the Principal Triangulation' of England, although the English observations are inferior to those of India. Their system of reduction is entirely different from that of the Ordnance Survey and they cannot modify against experience gained in England. John explains the differences in approach, thinking that India may have to abandon the included angle system as too laborious. John is more ready to do this than Walker. He asks his father to help him out with a difficulty, if this interests him: Indian Survey observations are never less than twenty from every station of the principal triangle and latterly thirty has been the minimum, explaining abstracted observations and values. He asks how to obtain a probable value and probable error, noting the assistance of Airy's book, but aspects of the Ordnance Survey method seem incomprehensible to him. He explans why and how it may be that the Survey is correct, but in the act of writing he sees a way to proceed. [From 17 April] He comments upon the mail, having received a letter within 29 days, the first occasion it has been on time. Fancy's [Francisca Herschel's] letter is cheerful and John discusses the family news therein, with Julia [Herschel] going to Hastings and his mother thinking of joining her, and Bella [Isabella Herschel] off to Boulogne. He has heard from Willy [William James Herschel] who is at Purneah [Purnia] near Darjeeling. |