Benjamin Franklin Papers
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Observations at Sea on Temperatures of Air and Water, 10 April 1775 to 3 May 1775

Observations at Sea on Temperatures of Air and Water

Printed in Transactions of the American Philosophical Society. . ., II (1786), 325.

On his voyage to America Franklin was wondering about the sea around him. Soon after setting down his speculation on the speed of ships (the preceding document) he, or perhaps his grandson under his direction, began to record the temperature of the water. For the first four days it varied little, and perhaps for that reason the readings were abandoned for the next eleven days. They were resumed on the 26th, and from the 28th were made at various times of day and evening, accompanied by other observations, until the ship was at or near the mouth of the Delaware. From these data the following table was compiled. The manuscript has been lost, and the table did not appear in print until more than a decade later.9 Hence we do not know when the data collected on shipboard were cast in their present form, but the form has no bearing on their significance. For years Franklin had been interested in the Gulf Stream.1 Now for the first time he had the evidence to detect it. “Besides the gulph weed with which it is interspersed,” he wrote years later in the light of this and subsequent evidence, “I find that it is always warmer than the sea on each side of it, and that it does not sparkle in the night.”2

[April 10 to May 3, 1775]

Observations of the warmth of the sea-water, &c. by Fahrenheit’s thermometer, in crossing the Gulph stream; with other remarks made on board the Pennsylvania packet, Capt. Osborne, bound from London to Philadelphia, in April and May 1775.

Date. Hour Temp. of
Air.
Temp. of
Wat.
Wind. Course. Distance. Latitude N. Longitude W. Remarks.
April 10 62
11 61
12 64
13 65
14 65 °  ′  °  ′ 
26 60 70 37 39 60 38 Much gulph weed; saw a whale.
27 60 70 S S E W b S 37 13 62 29 Colour of water changed.
28 8 A.M. 70 64 SW WNW 37 48 64 35 No gulph weed.
6 P.M. 67 60 34 Sounded, no bottom.
29 8 A.M. 63 71 N W 44 37 26 66 0 Much light in the water last night.
5 P.M. 65 72 N E Water again of the usual deep sea
11 dit. 66 66 N WbN W b s 57 colour, little or no light in it
30 8 A.M. 64 70 NE W b N 69 at night.
12 62 70 E b S 24 37 20 68 53 Frequent gulph weed, water continues
6 P.M. 64 72 ESE W b N 43 of sea colour, little light.
10 dit. 65 65 S 25 Much light.
May 1 7 A.M. 68 63 60 Much light all last night.
12 62 56 S S W W N W 44 38 13 72 23 Colour of water changed.
4 P.M. 64 56 W b N 21
10 dit. 64 57 S W WNW 31 Much light.
2 8 A.M. 62 53 18 38 43 74 3 Much light. Thunder-gust.
12 60 53 WSW NW 18
6 P.M. 64 55 NW WSW 15
10 dit. 65 55 N b W W bN 10
3 7 A.M. 62 54 30 38 30 75 0
[Note numbering follows the Franklin Papers source.]

9Incorporated as an appendix to BF’s “Maritime Observations,” for which see the preceding document.

1See above, X, 94; XV, 246–8.

2APS Trans., II, 315–16.

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