Papal Corvette “Immacolata Concezione”

National Library of Rome. Collection: Ceccarius

“Certainly one of the most attractive natural phenomena is the water color: sailing from Naples to Palermo everyone felt transported by admiration in contemplating that immense and marvellous expanse of azure waters.” But how is it possible to measure the water transparency? A standardized method was adopted at the end of the nineteenth century. This method, which lowers in the water a 30 cm diamater white colored disk until it disappears from the sight, was extensively studied by Alessandro Cialdi and Angelo Secchi and published in 1865 in the magazine “The New Cimento”. It is now universally known as the Secchi disk method.
For different reasons, sailors and explorers of the world were interested in the determination of water transparency; among these reasons there are the ones related to safety (no obstacles and dry), the search for fish and navigation around the iceberg by monitoring the change of color from the ship’s mast, and even to allow the recovery of lost goods at sea .
Prior to 1865 some experiments were carried on to measure the transparency of water, but it was Angelo Secchi to correlate the effect of the diameter of the disk dipped with the height of the Sun. Between April 20 and June 1, 1865 Angelo Secchi and Captain Alessandro Cialdi carried on a series of measures of water transparency “in the beautiful sea in front of Civitavecchia” onboard the Immaculate Conception. The latter was a steamship steamer of the naval fleet of the Papal States, built in 1859 by the Thames Iron and Shipbuilding Company. It was 54.5 m long, it was equipped with 8 brass guns and it could rely on a crew between 46 to 52 sailors. It was conceived for the Pope’s private use, starting with a pilgrimage to the Holy Land that Pius IX failed to accomplish due to the Italian Unitarian events. Later, with the conquest of Rome of 1870, the Pope ordered Cialdi to lead the ship to Toulon where he was disarmed in 1877.
From the Corvette and a speed boat, Secchi and Cialdi, helped by some men of the crew, carried out a series of measures of water transparency off to Civitavecchia until Terracina, using a 43 cm diameter disk , “the color of the dish was the white of the finer majolica “and one of 237 cm was prepared by arranging” the canvas from sails that were painted of gleaming white “. The measurements were made using various optical accessories: “A good binoculars… a Nicol prism, a tourmaline and a stack of crystal plates… a spectrometer and an aneroid barometer”. To assess the influence of the color other large canvas disks were made, painted with white, yellow ochre and “color of muddy sea background” oils. In the reports published in 1865, Cialdi and Secchi described the method used in a very clear way, as scientists trying to understand in depth the studied phenomenon in a systematic way. The applied method , for its simplicity, has become very common also for modern oceanographic expeditions.
Today there are even apps for smartphones ( and a world historical database of the data obtained with the Secchi disk(

Marcel R. Wernand, Royal Netherlands Institute for Sea Research
Oliver Zielinski, Institute for Chemistry and Biology of the Marine Environment, Università di Oldenburg

Boyce D. G., Lewis M. R., & Worm B. (2010), “Global phytoplankton decline over the past century”, Nature, 466, pp.591-596; Cialdi A. & Secchi A. (1865) “Sur la transparence de la mer”, 
Comptes rendus hebdomadaire de séances de l’Academie des Sciences, 61, pp. 100-104; Lee Z. P. & al. (2015), “Secchi disk depth: A new theory and mechanistic model for underwater visibility”, Remote Sensing of the Environment, 169, pp. 139-149; Riccò A. (1876), “Studi spettrali sul colore delle acque”, Memorie della Società degli Spetroscopisti, 5, pp. 101-115; Secchi A. (1864), “Relazione delle esperienze fatte a bordo della pontificia pirocorvetta Immacolata Concezione per determinare la trasparenza del mare”, Il Nuovo Cimento, 20, pp. 205-237; Wernand M. R. (2010), “On the history of the Secchi disc”, Journal of the European Optical Society: Rapid Publications, 5, 100135 1-6.