Tell me the sum of Gross Tonnage for isis ship on date commisioned less than 1842
CREATE TABLE "list_of_rmsp_company_vessels" (
  "ship" text,
  "date_commissioned" real,
  "gross_tonnage" real,
  "propulsion" text,
  "material" text
);
SELECT SUM("gross_tonnage") FROM "list_of_rmsp_company_vessels" WHERE "ship"='isis' AND "date_commissioned"<1842;