**1. Introduction**

The present study is part of the research line of industrial archeology whose purpose is the systematic study of the industrial memory of an era. Addressing industrial archaeology from the point of view of engineering provides a necessary vision for the correct understanding of industrial heritage, since in many occasions this study is carried out in an unscientific way, the written record of said heritage being incomplete and with the consequent risk of loss.

The work presented in this article has been developed within a research project on the work of an outstanding Spanish Enlightenment engineer, Agustín de Betancourt [1,2], analyzing his best-known inventions from an engineering graphics standpoint in order to obtain his geometric modeling [3–7].

As is well known, a first step for the recovery and study of historical technical heritage is the creation of realistic three-dimensional (3D) models. The article shows the 3D digital restitution of Betancourt's optical telegraph. This 3D model has been obtained using CAD (computer-aided design) techniques, following the objectives established in the document Principles of Seville [8] on virtual archeology which cites the London Charter [9] regarding the computer-based visualization of cultural heritage.

In 1791, the first telegraph using optical signals appeared in the work of the French Claude Chappe and his four brothers. It consisted of a dial similar to that of a watch with a moving hand that could adopt 16 positions, which each corresponded to a symbol. To indicate that the next telegraph station had received the message, it emitted an acoustic signal. However, the main drawback was in distinguishing precisely the exact position of the needle due to its size and the separation between symbols [10].

Subsequently, he developed an optical telegraph model based on five panels of the binary type (all or nothing) that assigned a code to each of the 32 possible combinations (25), but the results were not as expected.

Finally a third model was proposed which ignited the interest of the National Assembly of France and led to the Paris-Lille line coming into operation in 1794 along 230 km of telegraph line and 22 towers, the last one located in the dome of the Louvre, transmitting the first telegram in history [11]. Construction of the mechanisms that moved the telegraph relied on the knowledge of the Swiss watchmaker Abraham Louis Breguet, so the optical telegraph would come to be called Chappe–Breguet. The system was quite effective, although it faced the limitation that visibility between stations was not always optimal, a situation which caused numerous errors in transmission.

Simultaneously, Chappe's technological advances were widespread in numerous countries in Europe and the United States. Thus, in Sweden in 1794 Abraham Edelcrantz developed another type of telegraph that consisted of a system of 10 screens that could rotate 90º to be 'seen' or 'not seen'. Thus, the different positions of the screens formed combinations of numbers that were translated into letters, words or phrases through code books.

In the same year, 1794, the Englishman George Murray developed another telegraph, inspired by that of Edelcrantz, which consisted of six screens arranged around two columns, being able to form up to 64 possible combinations (26) to transmit messages [10].

In Spain the figure of the engineer Agustín de Betancourt stood out and during his stay in Paris he met Abraham Louis Breguet, with whom he could analyze in detail Chappe's telegraph. Between 1793 and 1796, Betancourt also traveled to London where he delved into Murray's telegraphic model. These visits allowed him to develop his own optical telegraph model [12], which improved on Chappe's telegraph in certain aspects.

Moreover, on 17 February 1799 King Carlos IV approved a project for the implementation of the optical telegraph in Spain, and in 1800 the first telegraph line that connected Madrid with Aranjuez began operating. However, the economic crisis that the country was going through did not allow Betancourt's invention to thrive [13].

After a review of the state of the art of the technical studies related to the Betancourt optical telegraph, only a single and interesting mechanical study has been found in the case that the telegraph stations are not aligned [14]. However, there is no study from the point of view of engineering graphics, that allows the reader to understand perfectly and in detail the operation of this historical invention that marked a milestone in the field of telecommunications. The main objective of this research is to obtain a reliable 3D CAD model of this historical invention that allows us to know in detail this outstanding engineering work. The originality and novelty of this research is that there is no existing 3D CAD model of this historical invention with this degree of detail, so it will help in an outstanding way in the detailed understanding of its operation. An educational goal is also pursued, through its exhibition on the websites of the foundations that have supported this research (Fundación Canaria Orotava de Historia de la Ciencia [15] and Fundación Agustín de Betancourt [16]), as well as in other museums of the history of technology. On the other hand, the impact of this research depends on its future uses. Among these, we can highlight:


• Printing in 3D using additive manufacturing together with an animation created by a photorealistic organizer for its exhibition in a museum, interpretation center, or foundation.

The remainder of the paper is structured as follows: Section 2 presents the materials and methods used in this investigation. Section 3 includes the main results of the process of geometric modeling and discussion of them in order to explain the operation of this device, and Section 4 states the main conclusions.

#### **2. Materials and Methods**

The material used in this research was obtained from two files on the Betancourt Digital Project website of the Canary Orotava Foundation for the History of Science [17], assigned for its digitalization by the National School of Bridges and Roads of ParisTech University. The first of these is manuscript 826 (MS 826), consisting of 3 sheets (Figures 1–3), and a descriptive memory of 26 pages written by Betancourt and Breguet [18] and divided into two parts: one dedicated to the explanation of the 3 sheets and the operation of the optical telegraph, and a second where the authors defend their ideas on how telegraphic language should be. The second is the 1806 (MS 1806) which contains several documents related to the optical telegraph [19].

**Figure 1.** Elevation (**left**) and profile (**right**) views of the optical telegraph by Agustín de Betancourt and Abraham Louis Breguet [18] (Courtesy of Fundación Canaria Orotava de Historia de la Ciencia).

**Figure 2.** Longitudinal (**top**) and cross (**bottom**) sections of the optical telegraph by Agustín de Betancourt and Abraham Louis Breguet [18] (Courtesy of Fundación Canaria Orotava de Historia de la Ciencia).

**Figure 3.** Observation angles and signal system of the optical telegraph by Agustín de Betancourt and Abraham Louis Breguet [18] (Courtesy of Fundación Canaria Orotava de Historia de la Ciencia).

The drawings used to model the telegraph appear with their respective graphic scales, which has greatly facilitated the correct dimensioning of each of the telegraph elements. The drawings, despite being plans as they are conceived today, are rich in constructive detail and surprise with their clarity. The possible doubts that have been generated, in some cases, by the lack of orthographic views are resolved thanks to the memory written by Betancourt, which details the operation and purpose of each element.

The first sheet (Figure 1) presents a general view of the elevation and the profile of the optical telegraph. In some parts, it omits details of the frame or support, which can hide important parts of the mechanism. The second sheet (Figure 2) shows a longitudinal and a cross section that highlight the relationship between the frames, showing in detail the transmission from one to the other by means of the transmission that joins the pulleys. The third sheet (Figure 3) is divided into two parts: the left part shows the elevation and plan views of the gimbal joint, while the right part illustrates the positions that the optical telegraph can take. The upper right part shows how the optical telegraph can be placed on a different work plane from the immediate telegraph stations (previous and rear), while the lower right part details the signs transmitted by the optical telegraph according to the direction the indicator arrow adopts.

Thanks to these drawings it has not been necessary to make assumptions about the geometry of the elements to be modeled, a rare aspect in other inventions of the same author, since the function of the plans at the time was to describe the function of the invention and not the to give details regarding its construction.

The research methodology has followed these steps:


The entire process described above is shown as a summative flowchart in Figure 4.

For CAD modeling tasks the software used has been Autodesk Inventor Professional [20] which has allowed us to obtain the 3D CAD models of each element, generating a file with extension (.ipt) and finally the assembly of the whole, generating a file with extension (.iam). Moreover, each element has been assigned material with certain physical properties in order to obtain a model that is closest to reality.

Three-dimensional CAD modeling techniques constitute a very important tool in the process of the study and design of historical heritage, for example, in the fields of cultural heritage [21], aerospace [22], civil [23], horological [24], architectural [25], and more generally, in the study of any virtual model [26–28], as a previous step to CAE analysis [29,30].

**Figure 4.** Summative flowchart of the methodology followed in the 3D modeling process.

#### **3. Results**

#### *3.1. Considerations and Functioning*

In order to give the reader a complete idea about the analysis of the invention of Agustín de Betancourt it is necessary to explain its operation. Figure 5 shows a plan of the ensemble with an indicative list of the different elements that form it which will serve to illustrate the operation of the mechanism, and Figure 6 shows an exploded view of the overall invention for a better understanding of the direction and order of assembly of the different parts of the optical telegraph.

**Figure 5.** Plan of the ensemble of the optical telegraph with an indicative list of all its elements and materials.

**Figure 6.** Exploded view of the 3D CAD model.

The operation of the Betancourt optical telegraph is based on the relationship between the indicator arrow (2) and the mast (3) with which it forms a certain angle easily visible over large distances. The directions that the arrow can adopt are taken over preset positions corresponding to the transmission code signs. The indicator arrow revolves around an axle that allows it to adopt any direction of an imaginary circle, dividing it into thirty-six sectors which correspond to numbers or letters of the alphabet. Comprising 36 divisions, each sector covers an angle of 10◦ sexagesimal: the first positions correspond to letters of the alphabet and the rest to ten figures (from 0 to 9).

Firstly, in order to change the position of the indicator arrow a manually operated main transmission (11) is necessary. In the lower part of the mast, there is a winch (9) with 36 slots, which was operated by the operator until the tip of said indicator arrow was placed on the slot with the mark of the alphanumeric character to be transmitted. In a similar way, the main transmission (11) communicates the movement of the winch (9) to the upper wheel (12) which is integrated with the axle of the indicating arrow so that turning the winch rotates the arrow, adopting a certain position. The aforementioned main transmission (11) is singular, since it is formed by two long pieces of hemp rope fastened at its ends by a chain of flat links joined by bolts, very similar to those used in current transmissions. Finally, the telegraph operator looks through the telescope's eyepiece (5) for the arrow position of the preceding telegraph to make sure that its arrow has the same position.

Each telegraph has two telescopes perfectly fixed in two wooden frames (6), which gives them stability and serves to keep them pointing in a certain direction (the one of the nearest telegraph: before or after). This telescope can rotate on its longitudinal axle without ever losing the direction of the immediate telegraph, and in addition it is housed in a pulley (7) arranged transversely to its main axle. This pulley (7) is also joined by a transmission to another pulley (8) joined in turn by a gimbal joint to the axle of the winch. This gimbal joint allows the telescopes to be rotated at the same time as the winch, thanks to a lower transmission (4) existing between them, similar to the main transmission (11) mentioned above.

Furthermore, the telescope's eyepiece is provided with a meridian wire that serves to show the position of the indicator arrow of the telegraph itself. Thus, if the operator observes that the indicator arrow of the preceding telegraph changes the winch will rotate until the position of its arrow is the same as that of the preceding telegraph. In the same way, the operator can see if the rear telegraph station indicates the position that he has transmitted and will not change position until it is transmitted correctly.

An important consideration is that telegraph stations do not have to be perfectly aligned for proper operation, that is to say that it is not necessary that the frames be arranged in parallel, since the gimbal joint allows the transmission work plane to be different from that of the circle formed by the indicator arrow.

Finally, the indicator arrow has three oil lamps (1) that allow the telegraph to continue operating at night, since they indicate the direction of the arrow in the dark, the operating mode being the same as in full daylight.
