*3.4. Discussion and Findings*

The optical telegraph presented here possesses a series of characteristics that made it the most advantageous telegraph of its time.

First, it is a telegraph with a relatively simple assembly. The mechanism used for its operation is very basic, although somewhat more complex than that of the Edelcrantz panels, but in comparison, the set of transmissions and telescopes greatly simplifies its handling, since only one operator is needed who does not have to know the signs he transmits.

**Figure 22.** Optical telegraph assembled with all its elements.

Secondly, the transmission method using only an indicator arrow makes the message clearly gain compared to the method of panels or two arms used by other telegraphs. The use of an indicator arrow allows the operator to use 36 positions, allowing him to use the conventional language of signs and numbers and not a complex network of codes.

Thirdly, the indicator arrow allowed the use of oil lamps to distinguish the signals at night, so as long as the visibility was adequate transmission of the message was not reduced to the hours of daylight.

On the other hand, the optical telegraph also presented a series of drawbacks that weighed down the life of the telegraph and that finally led it to disappear shortly after it was put into operation. Interestingly, among these drawbacks, only one is of a technical nature, and it was precisely the one that would go unnoticed at the time.

The use of telescopes entailed a small error that was not possible to avoid at the time. When sighting a telegraph at certain angles with a telescope (it has already been seen that it is not easy to accurately locate both the immediate telegraph plane and the observation plane), some optical deformation occurs in the reading. For example, for a 30◦ angle difference, the observation error is 4.6◦. The measuring instruments of the time, especially compasses, did not allow for more precision when aligning these planes, although a 30º offset would be an excessive assumption since the sectors for the same signal are 10◦. Therefore, it cannot be assured, that in any of the messages no mistake was made, although in general with that maximum error it would be perfectly acceptable.

On the other hand, the use of the gimbal joint could also generate some error in the transmission. In addition, at the time there were any mechanical studies on the movement of said gimbal joint, which is not homogeneous and, therefore, the transmission was not a synchronized movement between pulley (8) and winch (9). A study of the limits of the telegraph [14] provides more complete information on the subject, although the conclusion is that the error was negligible, partly due to the use of two gimbal joints for two telescopes.

The last drawback presented by the optical telegraph was the candle effect. A slender structure located in a geographical location with adverse atmospheric situations was subject to great stresses, although so were the structures that supported the rest of the telegraphs. To avoid this effect, Betancourt devised the indicator arrow as a lightened structure, that is, instead of being a solid wooden pole, it consisted of a wooden body, empty inside, but consistent thanks to the slats that reinforced its structure. This arrow was less resistant to wind and reduced stresses in the support structure.

However, the main problem faced by the telegraph was that it was a patented invention (Betancourt–Breguet), and the conditions of the patent (Breguet had the exclusive rights to its installation) were viewed suspiciously and judged by political interests. The installation of the telegraph was not a small cost, since it took a good number of telegraph stations to cover the routes. In Spain while the influence of Betancourt remained the optical telegraph project went ahead. Thus originally a first test line, Madrid-Cádiz, was to be implemented, although only the first phase of the Madrid-Aranjuez line was installed since the disagreements with Manuel Godoy eventually condemned the project. In France, the payment of a patent to a foreign company was the main problem they faced. From a technical point of view, Betancourt and Breguet repeatedly demonstrated that their invention had obvious advantages over the French Chappe telegraph, but Chappe's better personal contacts gave his invention the final advantage.

When modeling the invention in 3D, it was discovered that the transmissions of the optical telegraph were not made of rope, although the models that exist of the invention in some parts of the world have not taken this aspect into account.

This observation is not an insignificant matter since if Betancourt and Breguet had proposed a telegraph with a rope transmission it would have been necessary to calibrate it every so often. The mass of the arrow with its axle is 83 kg and therefore is not excessive as it is a lightened structure, but its resistance to the wind and the arrow's own inertia when moving would have led to the hemp rope sliding on the pulley, causing a not unsubstantial error which would need to be corrected from time to time.

Betancourt does not propose a chain transmission arbitrarily, but does so with a very clear intention. The use of a metal chain inserted into a wooden pulley (or in a wooden winch) and the tensor of this chain made the transmission translate its geometry onto the wood which acted as a mold. In this way there was no risk of the transmission sliding through the groove of the pulleys, so a correction that was costly to solve and that periodically required a large adjustment time was avoided.

Finally, the gimbal joint is another of the great contributions of the invention which demonstrates the great mastery of mechanics of the Spanish engineer. Its use facilitated the adaptability of the invention to geographical areas where there is a physical impediment for telegraph stations to be aligned, and furthermore, when using this articulation it was not necessary for the telescope frames to be parallel to the frame of the indicator arrow and they could therefore work on different planes.
