The portolans may not be the only unrecognized transmission from classical times.
Leonardo da Vinci (1452 - 1519) intensively used books as inspiration or blueprint for his technical drawings. That is not well-known and sometimes simply neglected. In the prominent publication of Reti (1974) the steam canon "Architronito" is presented (not by Reti) as an invention of Leonardo. But Leonardo wrote that it was an invention of Archimedes. The same numbers for projectile weight and range that Leonardo presented was found by Reti (1962) in a book by Robertus Valturius, "De Re Militari" (1472).
Either Leonardo had it from him or he and Valturius had the same classical source we no longer know. The 15th century was the high time of search for classical texts. Some survived in only a single edition. It is very likely that the 15th century had some classical texts that are no longer extant.
According to statistics about Leonardo's extant writings, some 6000 pages, Augusto Marinoni (1974) could calculate by two ways a loss of 50 and 75 %. So probably more than half of Lenoardo's writings are lost.
According to historic records his Codex Trivulzianus had
So the main loss happened in the 16th century. As the Madrid codices show, this loss was probably not random but systematic.
With that in mind, one should expect serious gaps in his extant writings and technical sketches. Especially the latter could be removed from the bulk as valuable economic or military secrets. It is therefore no valid argument to dismiss the following as odds by chance. It may be rather the peak of a historical issue waiting for excavation. In hindsight it seems unfortunate that the facsimile editions in the 1970s were published in less than 1000 copies. A broader public might have found more still unrecognized inventions.
There is a claim that in 1757 a Mr. Ximenes found a letter from Leonardo to Christopher Columbus. It dated to 1473 and was about the probability to reach India on the "intended" way. The date 1473 seems too early but would have some important implications about Leonardo and possibly Columbus too.
See Leonardo's Bicycle
It has a clearly visible compass indicator and was powered by a water wheel. The water wheel was not the overshot preferred on land but an undershot possible to install on a ship. Something that could resemble a cardanic mounted gyroscope is visible in the center under the compass. But the drawing focused on the wheels for power transmission to the central section.
The whole device only makes technical sense as a gyro compass because anything else would not need the power. But the crucial part of the power transmission to the spinning gyro is not visible. On early gyroscopes it was done using a strong airflow. The base below the gyro could be a wind bag. The whole device would be a simple direction indicator like the ones used on small aircraft today, not a north finding compass as used on larger ships.
The gyro itself is sketched rather simply. Cardanic mounts were known around 1500 and Leonardo had a sketch with one. This drawing may need another one with the mount details of the gyroscope. It is probably lost. Whether Leonardo already understood the principle of a gyro compass seems not certain. The drawing may be inspired or copied from an unknown source.
On folio 58v (Madrid I ?) Leonardo wrote that the air becomes rarer with altitude and has less resistance. That implies a lower air pressure with altitude and a vacuum beyond earth. It was not the knowledge of his time and a sensation when Pascal experimentally proved it in 1648.
The emergence of cannons after 1250 raised the interest in the shape of a projectile's path, called the ballistic trajectory. It was crucial for targeting. Since Albert of Saxony (c. 1320-1390) the trajectory was thought to be of three parts. First a straight line, then a circular arc because of gravitation, and than again a straight line to the bottom.
But Leonardo, in Codex Madrid I, folio 147r, suggested a parabola in c. 1493. It was against all knowledge and experience for a century before and after him. No one else had suggested it then. Galileo derived it from experimental analyses mathematically. There is no evidence that Leonardo did it that way or even had the knowledge to do it.
It is suggested that he got it by observation. Could Leonardo really be such a better observer than generations of bowmen and gunners before and after him? Due to air resistance a real ballistic trajectory is not a parabola. Depending on the projectile material it may be closer to what the military books suggested then.
The simplest explanation is a classical book on mechanics that Leonardo had but that we no longer know. The parabola is a Greek term from classical times and we have transmitted texts about it. It is not far fetched they had used the parabola for describing the ballistic trajectory. The rarer air with altitude may be from the very same source.
Cornelis Jacobszoon Drebbel (1572-1633) from Alkmaar, Netherlands, was a well known inventor then. He is credited with the invention of mercury fulminate, scarlet dye, the air cooler, the thermostat, the thermometer, the microscope with two convex lenses and the navigable submarine. If only partly true he was still one of the most important inventors of his time. But two of his inventions seem beyond his time.
In 1662 the famous scientist Robert Boyle wrote in his diary about a conversation with "an excellent mathematician", who was still alive and had been on the submarine. He reported Drebbel's opinion that only a part of the air was necessary for breathing. After that quintessence part was consumed the "flame of live in the heart will go out". Drebbel was able to get this quintessence for life by a "chymical liquor" he had stored in bottles in the submarine.
Other reports indicate Drebbel was heating nitre that generated oxygen. But it generated potassium oxide or hydroxide too. That is able to remove carbon dioxide from the air. So he made a crude rebreather nearly 300 years earlier. But the air had to flow through the bottles by some pump.
Some points of this story seem beyond Drebbel's abilities. He correctly stated that only a part of the air (today called oxygen) supports life and can be replaced. But that was against the science of his time which offered a simpler explanation.
His correct explanation could only be created by experiments and logical deduction. But there is no record Drebbel ever did that. Had he, he would be praised as a high scientist-founder like Galileo and not just an inventor. It seems he had knowledge that did not originate with him and he did not know how it was created.
For his submarine not the oxygen generation was crucial but the removal of carbon dioxide by the rebreather. Again Drebbel had no explanation how it worked and was probably not aware of it. The rebreather was more difficult to develop than an oxygen generator. Because as an experimental indicator it needed the use of humans, not just a candle.
There is no direct way to build a rebreather by chance. Unlike an oxygen generator that releases a gas, one has to pump the air through the rebreather to scrub the carbon dioxide. He indicated no knowledge that he had to remove some gas. So how could that machine have happened by chance? It seems more likely he got both concepts from an unknown source he never mentioned.
That it really worked is attested in a 1615 letter by Johannes Kepler (1571-1630). Drebbel was with his clock at the court of Emperor Rudolf II in Prague where Kepler was then the chief mathematician. Kepler was the leading physicist of his time and wrote about Drebbel's perpetuum mobile: "it is good, it convinced me, it finds my approval".
But neither Kepler, nor Drebbel nor anyone else had an explanation how it worked. The barometric pressure was unknown then until Torricelli measured it in a secret experiment in 1643 using a mercury tube. And the discovery of the daily variation still had to wait an additional 20 years for a better barometer.
It is strange to imagine that Drebbel and Kepler had an operational pressure measurement device in use without realizing it. The pressure oscillations are invisibly small on a sealed clock body. It had to have an ingenious lever mechanism to record it. It took until 1770 for the London watchmaker James Cox to build a barometric clock again.
Any discovery of this clock by chance seems highly unlikely. If Drebbel really invented it, he had to have a barometer. Only from an accurate barometer it was possible to deduce and design the clock.
In all three cases - the oxygen theory, the rebreather and the barometric clock - Drebbel had knowledge that could only be achieved with considerable experimental effort and quantitative analysis. The clock even needed a barometer before. There is no evidence or hints that he conducted the necessary experiments or was even aware of them. Instead he had no knowledge of air pressure and did not know that his rebreather removed a gas.
At his time there is no other source known that could have provided the necessary knowledge. The most recent epoch with the ability to do that was in classical times. By developing window glass and concrete they had proven skills for an experimental quantitative approach.
Main source: Clausing, Achim: "Cornelis Drebbel: Ein vergessener Pionier der Moderne", Spektrum der Wissenschaft Juli 2011, pp. 88-95
It is common opinion that the first telescope was invented around 1606 in the Netherlands and nobody had anything like it before. Robert Temple did an extensive investigation on this subject and found a lot of evidence to the contrary. Dozens of lenses in museums from archaeological digs support his view.
The earliest telescope mentioned in texts belong to the Pharos lighthouse tower of Alexandria at the time of Ptolemy III Euergetes (c. 280-221 BC). It could see ships far away and was first mentioned by Giovanni Battista della Porta in his famous book "Magia Naturalis" 1558. According to Temple, Porta then goes on to describe the construction of a telescope, many years before Galileo. In 1763, Bonaventure Abat offered further sources, one possibly Arabic. He concludes that the mentioned object was a mirror, not a lens. This was supported by Count Guillaume Libri in 1835 with several Arabic sources.
In recent times the opinion came up that the Pharos was not built to guide ships in the harbor and had no fire atop. The fire to guide ships was from a later time after the Mediterranean Sea was more secure. This opinion together with the telescope accounts suggests the Pharos was an early warning station against raider fleets. It could allow sufficient warning time to allow Alexandrian forces to await the raiders in their most vulnerable stage - at the water line. That would explain well why such an expensive, monumental building was created at all. Lighthouses were then and today always smaller.
Temple found a lot more references to ancient telescopes. But one stood out in a way that may connect classical, medieval and early modern references. It was by the early scientist Roger Bacon (c. 1214-1292). He wrote in his 1267 Opus Majus:
How credible is it that Caesar in 54 BC had a telescope? It is true that a mirror set is able to work as a telescope. It may even be superior to a lens telescope of the 17th or 18th century. Because of color distortions, lens telescopes could not rival good mirror systems for a long time. But the latter were clumsy and needed to be polished often to keep a bright image at night.
For a stationary daylight observation, a mirror system was a better choice but rarely used. The mirrors would not be "very large" to get good magnification but needed an exact curvature and alignment. About 100 mm diameter would be enough. A role for larger mirrors seems doubtful.
Still in the 16th century Bacon's words were ridiculed because mirrors could not enlarge the view. Today we know better. But Bacon could not know it, he had to have received it from an ancient source. The passage with Caesar is not in our transmitted texts. So Bacon had access to another version we no longer have.
It seems Bacon was in possession of a text that described how a lens telescope could be built. The 1911 Encyclopædia Britannica wrote about the passage in his Opus Majus: "He certainly describes a method of constructing a [lens] telescope, but not so as to lead one to conclude that he was in possession of that instrument."
According to the Encyclopædia Britannica "about 1257, Bonaventura, general of his order, interdicted his lectures at Oxford, and commanded him to place himself under the superintendence of the body at Paris. Here for ten years he remained under supervision, suffering great privations and strictly prohibited from writing anything for publication."
That was circumvented by an order of pope Clement IV. (1265- 1268) for whom he wrote 1267 the Opus Majus and an unknown number of other books. In 1278 his books were condemned by Jerome de Ascoli, general of the Franciscans, later to become Pope Nicholas IV., and he was thrown into prison for fourteen years. He died soon afterwards. The oldest known reference to the imprisonment of Bacon is from around 1375. It says the order was given by the head of the Franciscans because of unspecified "suspected novelties".
In his Opus Majus, Bacon was the first scientist who described the use of lenses and optics. He only had to put two different lenses together to get a telescope or a microscope.
In 1551 - well before the official invention of the telescope - the English polymath Robert Recorde wrote in his book "The Path-way to Knowledg, Containing the First Principles of Geometric" about Friar Bacon:
According to Recorde, Bacon was then accused of being a black magician. But Recorde thought it may only have been applied mathematics, geometry.
Due to the availability of lenses then it seems almost certain that Bacon built a telescope and made observations like Galileo did 330 years later. The mountains on the Moon and the moons of Jupiter are visible even with the crudest telescope. Neither was in agreement with the then prevailing theology and an improvement of the instrument would have made the conflict even worse.
Bacon mentioned a classical book about secret inventions written in Greek. Bacon's extant writings are said to contain information about steam machines, self-moving wagons, diving equipment, flying machines and gunpowder.
With three writers (Leonardo, Drebbel, Bacon) knowledge was identified that could not originate from their time. Bacon mentioned an event in 54 BC that is unknown otherwise and he mentioned a secret book that could be the way of transmission.
The machines Bacon mentioned had some cross match with those of Leonardo. Leonardo was with the most powerful people of Italy and therefore well-connected. We don't know his sources but it could well include the very same book Bacon mentioned.
The first globe gores with Antarctica were drawn by Leonardo and are a striking link to the other traces of a classical portolan world map. So Leonardo had more classical material than Bacon and probably from different sources - not just one book. But no copy of a classical source text survived to present times by way of any of the three writers.
Given the availability of lenses since the 13th century it seems certain that some people between 1300 and 1600 built simple telescopes. But no direct report about it or their astronomical observations survived. Because of the expected large public impact - as Galileo had in 1609 - this is a strong indicator for a systematic and organized suppression.
Like Bacon was called a black magician, others may have faced that accusation, too. The Secret Courts were tasked to find any person suspected of black magic. The accused were never heard and the only way of punishment was to kill them. The silence until Galileo suggests this organization was still active after their last record of 1490.
I thank Bibhistor for suggesting this page and crucial contributions.