The site: Blue Hill Meteorological Observatory is, according to Wikipedia, "the foremost structure associated with the history of weather observations in the United States. ... It is home to the oldest continuous weather record in North America, and was the location of the earliest kite soundings of the atmosphere in North America in the 1890s, as well as the development of the radiosonde in the 1930s. ... The observatory remains active to this day, continuing to add to its data base of weather observations now more than one hundred years old, and stands as a monument to the science of meteorology in the United States."
The postcard: This never-used card might be pre-1907, as it does not have a divided back. It is marked as E.9026 and was published by "The Metropolitan News Co., Boston and Leipzig, Manufacturers of Souvenir Postal Cards." Metropolitan was in business from 1905 until 1916, according to the (unrelated) Metropolitan Postcard Club of New York City. That website states that it was "a major publisher and printer of view-cards in color, black & white, sepia, and with hand coloring in both halftones and in collotype. They captured views throughout the American Northeast but postcards of New England scenery were produced in greatest number. Many of their cards have a heavy look resembling early chromolithographs."
The Sea Wall, St. Augustine, Fla.
The site: St. Augustine, Florida, is one of the oldest cities in the United States. And so it would naturally follow that its seawall is quite historic, also. According to the website of The Avenida Menendez Seawall Project: "The historic St. Augustine Seawall has long been an integral part of the city’s fabric — a coquina sentinel from rough waters, a promenade for romantic strolls and waterside socials. The Spanish first began building the original seawall in 1696 south of the Castillo de San Marcos fort. The section now being improved was built between 1833 and 1844 by graduates of the United States Military Academy at West Point. But it has been battered by time and tide, from a devastating hurricane in 1846 that partially collapsed it to Tropical Storm Fay in 2008."
The postcard: This never-used postcard also has a back that is for the address only, making it pre-1907. The small type on the front states: "E.C. KROPP, PUBL., MILWAUKEE NO. 1640." According, again, to the Metropolitan Postcard Club, that company "began producing chromolithographic souvenir cards and private mailing cards in 1898 under the name Kropp. These cards were of much higher quality than those that would printed under the E.C. Kropp name. They became the E.C. Kropp Company in 1907 and produced large numbers of national view-cards and other subjects."
On a final note, the closest building in this postcard has a sign stating "THE VEDDER COLLECTION." Underneath that, a small sign states "MUSEUM FOR [?] ST. AUGUSTINE INSTITUTE OF SCIENCE AND HISTORICAL SOCIETY."